Delayed Hominization

A Thomistic defence

Introduction.

Much of the common discourse on St. Thomas’s and the Thomistic school’s views on delayed hominization (also known as delayed animation or delayed rational ensoulment) has shown that many people don’t properly understand the great Doctor’s position or the position of his school. Most of these discussions centre around the debate of abortion, with the general thought being that if delayed hominization is true then some abortion must be permissible, and if immediate hominization is true then no abortion is permissible. This characterisation is obviously overly simplistic as many, including myself, quite easily hold the position of delayed hominization and that no abortion is permissible. Yet, it brings up the question of when rational ensoulment happens, and specifically among Thomists who have to contend with St. Thomas’s position of delayed rational ensoulment. Many contemporary commentators discuss the Thomistic position without properly understanding it, and this leads them to the conclusion that if St. Thomas had the knowledge of modern embryology today, he would have believed in immediate hominization (immediate animation or immediate rational ensoulment). We can see this from lay commentators like Catholic Answers[1] and from professional commentators like Sister Renée Mirkes, OSF.[2] There are even some academics who deny that St. Thomas even held to delayed hominization.[3] Tim Staples, from Catholic Answers, goes so far as to say that immediate hominization is taught at the level of the Ordinary Magisterium.[4]

Yet, many of these commentators miss the wider argument among scholastic Catholics and especially within the Thomist school around the position of delayed hominization. The purpose of this article is not to speculate on what St. Thomas’s position would have been if he had access to today’s scientific knowledge. Nor is it to be a comprehensive overview of all the literature around delayed hominization. Rather, it is to help a lay reader with a decent knowledge of philosophy understand the position of the Thomistic school on delayed rational ensoulment and to show that it is a very probable position that can be rationally defended today.

Let us begin by remembering the words of Pope St. Pius X, “We admonish professors to bear well in mind that they cannot set aside St. Thomas, especially in metaphysical questions, without grave detriment”.[5]

The structure of this article will be as follows:

1. Explaining the methodology of answering this question.
2. Providing a brief history of this question.
3. Providing definitions of key terms.
4. Looking at the question from the perspective of theology.
5. Looking at the question from the perspective of philosophy.
6. Looking at the question from the perspective of the empirical sciences.
7. Responding to Objections.
8. Conclusion.

1. Methodology.

In the debate of when hominization occurs, there appears to be two distinct methodologies. These two methodologies can be seen in the difference between Joseph F. Donceel S.J.’s article ‘Immediate Animation and Delayed Hominization’[6] and Benedict Ashley O.P.’s chapter ‘When Does a Human Person Begin to Exist?’.[7] In the first methodology, the method begins with looking at the philosophical principles and then sees how they apply to the scientific facts. The second methodology begins with the scientific facts and then shows how they can be explained by philosophical principles. This may seem like a minor difference, but where you start from can affect where you end up. A note then should be made as to what principles are more certain and what are the different competencies of areas like theology, philosophy, and empirical science in addressing this question.

Theology is the science of God and God’s relationship with His creation from God’s perspective. This is separated into natural theology which investigates what we can know about God from natural reason and what is called natural revelation, and sacred theology which investigates the supernatural revelation that God has given mankind. Natural theology is a part of philosophy and specifically a part of metaphysics (the science of immobile being), which deals with the most fundamental principles of reality and per se grounds the principles we find in the philosophy of mobile being (often called the philosophy of nature). We would say that the philosophy of nature is a subaltern science to metaphysics. However, Sacred Theology, is the highest science and all other sciences are subaltern to it, because its principles are most certain, since they come directly from God who can neither deceive nor be deceived, and because they deal with the most noble subjects (i.e. the mystery of the Trinity). Since God has given us a teaching authority (magisterium) that has the ability to define these theological principles which He has promised to safeguard, we can be sure that what this magisterium hands down to us as part of the deposit of faith is most certain.

Philosophy is the science of things through their highest causes, acquired by the natural light of reason. As a science it is a certain and evident knowledge acquired through demonstration. It is a certain knowledge which distinguishes it from opinion, which can be erroneous. It is an evident knowledge, which distinguishes it from faith, which is not evident (as it is unseen) but is accepted through signs of credibility. Finally, it is a knowledge acquired through demonstration, namely that the thing is known through its causes, as a thing is not properly known except through its causes. The causes by which a thing is known are the four causes, formal, material, efficient and final. However, it isn’t just any knowledge through the four causes, but the highest order of knowledge, one beyond which someone can’t ascend. Since it is impossible to ascend higher than being, it is the being of things that satisfy the intrinsic causes in philosophy. For the extrinsic causes, since we can’t ascend past the first efficient cause and the ultimate final cause, these satisfy the extrinsic causes in philosophy. Finally, this science is acquired through the natural light of reason, which distinguishes it from Sacred Theology. Since Philosophy pursues that which is highest in the natural order, it gives its principles to every other natural science, and thus every natural science is subaltern to philosophy. The principles of philosophy, therefore, are more certain than the principles of empirical science since empirical science depends on philosophy to make its principles knowable.

The empirical sciences are specified by their formal object. The formal object of an empirical science provides the principle/s under which it is dependent. For example, Biology is the science of life and has living organisms as its material object, but biology itself doesn't determine what life essentially is, as this definition serves as one of its principles received from the philosophy of nature. Biology studies living things under a particular aspect (formal object quo) - their observable, measurable properties and operations - but cannot determine through its own method the essence of life itself, since this essence serves as a principle for biological investigation. As a subaltern science, biology depends on philosophy not only for its definition of life but also for the justification of its first principles and the intelligibility of its conclusions.

Objection 1: There is greater disagreement in metaphysics than there is in the empirical sciences. But where there is greater disagreement there is less certainty. Therefore, there is less certainty in metaphysics than there is in the empirical sciences.

Response: I distinguish the minor: where there is greater disagreement there is less certainty to us, conceded, less certainty simply, denied. Therefore, I similarly distinguish the conclusion: there is less certainty to us in metaphysics than in the empirical sciences, conceded, less certainty simply, denied.

First part is proved: that science is more certain simply which (a) makes us know the cause and propter quid (on account of what) more perfectly, and (b) whose object is more abstract from matter. But metaphysics excels in both: first, it proceeds through the highest and first causes in being itself, thus showing the propter quid most perfectly; second, its object is most abstract, for it considers things abstracted from all matter, even intelligible matter, having positive immateriality. Therefore, metaphysics is more certain simply.

Second Part is proved: That science is more certain to us which is more easily applied to the mind and more quiets the intellect. But sensible things about which the empirical sciences discuss are more easily applied to the intellect than abstract and invisible things about which metaphysics is concerned. Therefore, as to us the empirical sciences are more certain than metaphysics.

This distinction between what is more certain simply and what is more certain to us characterises the distinction in the methodology between Donceel S.J. (who started with what was more certain simply), and Ashley O.P. (who started with what was more certain to us). However, since the empirical sciences depend on the principles of philosophy and theology, it is better to start with what is more certain simply, i.e. theology and philosophy.

Objection 2: Any debate that deals with changes to a biological organism is a matter for the empirical sciences, not theology and philosophy. However, the debate over delayed/immediate hominization is a matter that deals with the changes of a biological organism. Therefore, the debate over delayed/immediate hominization is for the empirical sciences to determine, not philosophy and theology.

Response: I distinguish the major: Any debate that deals with changes to a biological organism is a matter for the empirical sciences alone if it concerns only material changes, conceded; if it concerns substantial changes involving immaterial or formal principles, denied. I similarly distinguish the minor: The debate over delayed/immediate hominization deals with changes of a biological organism that are purely material, denied; that involve the infusion of an immaterial soul, conceded. Therefore, I deny the conclusion.

The distinction is proved: Hominization precisely concerns when a human soul, which is the substantial form of the human body, is infused into matter. But the human soul, being intellectual and spiritual, transcends material conditions and thus cannot be detected by empirical methods alone. Therefore, while empirical science can tell us about the material changes in embryological development, it cannot alone determine when hominization occurs, as this requires philosophical principles about form and matter, and theological principles about the human soul.

Therefore, the methodology of this article should be clear. We will start with a brief history of the debate. Then we will provide some definitions. Then we will look at the theological principles and definitions handed down by the Magisterium as these are most certain. Then we will discuss the philosophical principles of the human soul and its union with the body which directly relate to hominization. Finally, we will consider the empirical sciences of embryology and how we can use these facts to explain the Thomistic position.

2. A Brief History of the debate.[8]

It is very clear that the embryology at the time of Aristotle and St. Thomas is very different than the embryology that we have today. However, this debate has continued regardless of the scientific understanding of the development of embryos.

Aristotle is the first to bring up the question of when hominization occurs. He discusses it in his Generation of Animals book 2, chapter 3 where he says,

It is not only necessary to whether what is forming in the female receives anything material, or not, from that which has entered her, but also concerning the soul in virtue of which animal is so called (and this is in virtue of the sensitive part of the soul)‒does this exist originally in the semen and in the embryo or not, and if it does whence does it come? For nobody would put down the embryo as soulless or in every sense bereft of life (since both the semen and the embryo of an animal have very bit as much life as a plant), and it is productive up to a certain point. That then they possess the nutritive [vegetative] soul is plain (and plain is it from the discussions elsewhere about soul why this soul must be acquired first). As they develop they also acquire the sensitive soul in virtue of which an animal is an animal,… For e.g. an animal does not become at the same time an animal and a man or a horse or any other particular animal. For the end is developed last, and the peculiar character of the species is the end of the generation in each individual. Hence arises a question of the greatest difficulty, which we must strive to solve to the best of our ability and as far as possible. When and how and whence is a share in reason acquired by those animals that participate in this principle? It is plain that the semen and the embryo, while not yet separate, must be assumed to have the nutritive soul potentially, but not actually, until (like those embryos that are separated from the mother) it absorbs nourishment and performs the function of the nutritive soul. For at first all such embryos seem to live the life of a plant. And this is clear that we must be guided by this in speaking of the sensitive and the rational soul. For all three kinds of soul, not only the nutritive, must be possessed potentially before they are possessed in actuality. And it is necessary either that they should all come into being in the embryo without existing previously outside it, or that they should all exist previously, or that some should so exist and others not. Again, it is necessary that they should either come into being in the material supplied by the female without entering with the semen of the male, or come from the male and be imparted to the material in the female. If the latter, then either all of them, or none, or some must come into being in the male from outside.
Now it is impossible for them all to pre-exist is clear from this consideration. Plainly those principles whose activity is bodily cannot exist without a body, e.g. walking cannot exist without feet. For the same reason also they cannot enter from outside. For neither is it possible for them to enter by themselves, being inseparable from a body, nor yet in a body, for the semen is only a residue of the nutriment in process of change. It remains, then, for the reason alone so to enter and alone to be divine, for no bodily activity has any connexion with the activity of reason.[9]

Aristotle argues that animals proceed from acquiring a nutritive or vegetative soul to a sensitive soul after they have developed. Then Aristotle says that the rational soul is unique in that it seems to come to the body from outside and its source is divine and that man only receives this after they have developed the sensitive soul.

The Greek fathers generally held to immediate rational ensoulment such as St. Gregory of Nyssa[10], St. Basil of Caesarea[11], and Abbot St. Maximus[12], with Theodoret being an exception.[13]

The Latin fathers who mention the problem defend delayed hominization with some academics speculating that it was in reaction to Tertullian’s views on traducianism or the view that the human soul of the child comes directly from the parents. The Latin fathers argued that the human soul is created directly by God and distinguished between conception and animation (or what we would now call rational ensoulment or hominization). Some of the Latin fathers who support delayed hominization are Gennadius[14], St. Jerome[15] and St. Augustine[16].

The medievals were less split on the issue. Vincent of Beauvais[17] supported immediate hominization, whereas St. Anselm[18], Peter Lombard[19], Hugh of St. Victor[20], St. Albert the Great[21], St. Thomas Aquinas[22] and Bl. Duns Scotus[23] supported delayed hominization.

Delayed hominization remained popular in the following centuries and even Dante expressed the position in his Purgatorio. [24] However, in the 17^th Century when scientists like Thomas Fyens started promulgating the biological theory of preformationism, immediate hominization began becoming more popular again. Preformationism is the theory that organisms develop from miniature versions of themselves and their development consists in growing. This is contrasted to epigenesis which states that an organism forms from a seed, spore or egg and develops through a series of stages where cells differentiate and organs begin to form and mature. Aristotle and most Aristotelians such as St. Albert the Great and St. Thomas were epigenesists.[25]

St. Alphonsus Ligouri writing in the 18^th Century said that the prevailing opinion of his time was immediate hominization but still defended delayed hominization.[26]

Preformationism was largely refuted by embryologist Caspar Friedrich Wolff in 1758, but immediate hominization still remained popular with some academics pointing to the prevailing Cartesian idea of the soul as the main reason.

In the 1820s many of the modern theories of embryology were being formulated by the works of von Baer and Christian Pander who discussed the differentiation of the germinal layers and discovered the mammalian egg. By the end of the 19^th century, Wilhelm Roux was studying the separation of the blastomeres, Kölliker had described the development of the embryo in light of cellular theory, and delayed hominization was still being defended by Liberatore, Zigliara, Cornoldi, Lorenzelli[27], Sanseverino, and Di Maria.

In the first half of the 20^th century, there were many defenders of delayed hominization in light of the advances in embryology. De Becker S.J.[28], P. Vallet[29], P. Pégues O.P.[30], Lottini O.P.[31], Guidi O. Cap.[32], Cardinal D. Mercier[33], P. Marcellus of the Child Jesus, O. Carm.[34], V. Remer, S.J.[35], A. D. Sertillanges O.P.[36], D. Prümmer O.P.[37], Mattiussi S.J.[38], A. Farges-D. Barbedette[39], A. Vermeersch S.J.[40], B. Merkelbach O.P.[41], A. Pirotta O.P.[42], C. Carbone[43], F. X. Maquart[44], R. Jolivet[45], A. Lanza[46], E. Messenger[47], R. Lacroix[48], H. Grenier[49], and M. Barbado, O.P.[50]

However, in the second half of the 20^th century immediate hominization was widely accepted among Catholics due mostly to the practical concerns of abortion and other important bioethical issues. There were still many that made great defenses of delayed hominization such as Joseph Donceel S.J. in 1970[51], William Wallace O.P. in 1995[52], and some in the 1990s in the Theological Studies journal. However, most arguments in this second half of the 20^th century were infected with a poor methodology of trying to answer the question purely from the empirical sciences, or were trying to answer “What St. Thomas would have thought if he had our scientific knowledge” and thus their arguments were of a significantly lower quality since they weren’t addressing the important philosophical questions.

Now, in the 21^st century, the divide between immediate and delayed hominization among Catholics tends to follow the position the Catholic has on abortion, with the exception of a few scholastic philosophers who maintain the Thomistic position of delayed hominization and opposition to abortion.

3. Definitions.

Let’s start by defining terms that will be helpful for our discussion.

Soul is the first act of a physical, organic body having life in potency.[53]

First (or substantial) act. The soul is the principle by which living things are distinguished from non-living beings. Since living things are distinguished from non-living beings essentially and not accidentally, then the soul is not an accidental act, but an essential or substantial act (i.e. first act). Furthermore, the first act of a thing is its form, therefore the soul is a substantial form. However, it isn’t just any substantial form, but the form of a physical body that has life in potency. This distinguishes it from other forms such as subsisting forms, elemental forms, artificial forms, and mathematical forms.

of a physical… body. The word body can be understood in two ways; the first refers to prime matter itself which is immediately capable of a corporeal form, and this understanding wouldn’t distinguish a soul from other forms. The second way the word body is understood is matter formed by a grade of corporeity. Although the soul does confer corporeal being immediately on matter, if it is considered as a soul formally, it supposes in matter a grade of body that it itself bestows. In this way the soul can be considered as a form that bestows corporeal being, and reduplicatively as furnishing a body with animated being. It is in this second way that we consider the soul as the first act of a body since matter is conceived as constituted in the notion of a body simply, before being constituted in the notion of a living body.

organic. There are two things which are required for the notion of an organised body; 1) that the parts of the body be animated through the information of the soul. 2) that the parts be dissimilar or heterogeneous, so that they are distinguished from a mineral body. From this we can say that the organisation of the body can be taken accidentally and substantially. Taken accidentally, it refers to the disposition of organic parts to receiving quantity, figure or other qualities. It is clear that organic parts differ accidentally among themselves, since they are affected by different accidents. The organisation of the body taken substantially refers to the parts of the body being substantially different, not as though they each had a different substantial form (since there is only one substantial form in one composite), but that the substantial information of the same soul affects different parts differently which makes the bones, different to the brain, and the heart, etc.

having life in potency. This can be taken in a couple of ways. The first is the proximate potency to substantial life, such as the organised body is understood to be in a proximate disposition to receiving a vital form, which distinguishes it from a corpse which is only remotely disposed to the reception of a soul. However, commonly and properly ‘having life in potency’ is considered in a second way, which is accidental life, or vital operation which is life in second act. This vital operation is still life in potency even to an animated body which is life in first act.[54]

Vegetative Soul is the radical principle of nourishing, increasing, and generating vitally.

Radical principle. Radical (coming from the word ‘root’ or in latin radix and meaning ‘the origin’) principle since a substance doesn’t act immediately but from it flow forth the three aforementioned faculties which are the proximate principles of nourishing, increasing, and generating.

nourishing. Or the nutritive power is the faculty by which a living thing can conserve itself, repairing lost parts through conversion of food into proper substance.

increasing. Or the augmentative power is the faculty by which a living thing can acquire greater quantity for itself from food.

generating. Or the generative power is the faculty by which a living thing can produce another similar in species conjoined.

Sensitive soul is the first principle by which a being exists, lives and has sensation i.e the soul of an animal.

sensation. Sensation refers to both sensitive knowledge and sensitive appetite and depends on both internal and external sense organs.

Intellective (or Rational) soul is a spiritual substance which is the form of a body, created by God, to His image, for eternal beatitude.

spiritual. Subsisting independently of body, which supposes simplicity and immortality.

form of a body. A body containing vegetative and sensitive faculties.

created by God. God is the efficient cause of the rational soul (against traducianism).

for eternal beatitude. The final cause of the rational soul.

4. Theological perspective on the debate.

The Magisterium has made many pronouncements on the human soul and its origins.

The Fourth Council of Constantinople (869-70) in its canons says, ‘Even though the Old and New Testaments teach that man has one rational and intellectual soul, and all the Fathers and Doctors of the Church, who are spokesmen of God, affirm the same belief, some individuals think that man has two souls, and they reinforce their heresy by irrational demonstrations’ [DH 657].

The Council of Vienne (1311-12) in it’s constitution Fidae catholicae said the following,

‘Furthermore, with the approval of the holy council We reject as erroneous and contrary to the truth of the Catholic faith any doctrine or opinion that rashly asserts that the substance of the rational and intellectual soul is not truly and of itself the form of the human body or that calls this into doubt. In order that the truth of the pure faith may be known to all and the path to error barred, We define that from now on whoever presumes to assert, defend, or obstinately hold that the rational and intellectual soul is not of itself and essentially the form of the human body is to be censured as heretic.’ [DH 902]

Pope Benedict XII in his Cum dudum (1341) listed as an error that ‘the human soul of the son is propagated from the soul of his father, as the body from his body’ [DH 1007].

The Fifth Lateran Council in its Bull Apostolici regiminis (1513) says the following,

‘with the approval of the council We condemn and reprove all those who assert that the intellectual soul is mortal or that it is one and the same in all men or who raise doubts about this matter. The intellectual soul is not only truly, of itself and essentially, the form of the human body, as it is stated in the canon of Clement V, Our predecessor of blessed memory, issued by the Council of Vienne, but it is also immortal and, according to the great number of bodies into which it is individually infused, it can be, must be, and is multiplied’ [DH 1440].

Pope Innocent XI in his Sixty-Five Propositions Condemned (1679) condemned the following position of the Laxists:

It seems probable that every fetus (while it is in the womb) lacks a rational soul and [only] first begins to have [a rational soul] at the time it is born: as a consequence, it must be said that homicide is never committed in any abortion. [DH 2135]

In 1914, the Sacred Congregation of Studies in a decree approved the following Thomistic Theses:

Thesis 13. Bodies are divided into two parts: some are living, others lack life. Among the living [bodies], the substantial form, called the soul, demands organic disposition, or heterogenous parts, in order for there to be independently in the same subject a moving part and a part moved. [DH 3613][55]

Thesis 15. On the other hand, the human soul subsists by itself. When it can be infused into a subject that is sufficiently disposed, it is created by God, and by its nature it is incorruptible and immortal. [DH 3615][56]

Pope Pius XII in his encyclical Humani generis (1950) says, ‘for the Catholic faith obliges us to hold that souls are immediately created by God’ [DH 3896].

Other than these Magisterial pronouncements, there have also been Church documents that seem to support delayed hominization.

The Catechism of the Council of Trent (1566) says the following,

‘But something which goes beyond the order of nature and beyond human intelligence is the fact that, as soon as the Blessed Virgin gave her consent to the Angel's words... at once the most holy body of Christ was formed and a rational soul was joined to it. Nobody can doubt that this was something new and an admirable work of the Holy Spirit, since, in the natural order, no body can be informed by a human soul except after the prescribed space of time.’[57]

In 1588 Pope Sixtus V published the Bull Effraenatam where he makes the following distinction in his excommunication of all those who, in anyway whatsoever, brought about, ‘an abortion, or the expulsion of an immature fetus, whether animated or not animated, whether formed or not formed.’[58]

In 1591, in response to his predecessor, Pope Gregory XIV in his Bull Sedes apostolica said that, ‘where no homicide or no animated fetus is involved, not to punish more strictly than the sacred canons or civil legislation does.’[59]

In 1713, the Holy Office gave the following answer to a submitted question: “In the case under consideration [the baptism of an aborted fetus], if there is a reasonable foundation for admitting that the fetus is animated by a rational soul, then it may and must be baptized conditionally. If, however, there is no reasonable foundation, it may by no means be baptized…”[60]

However, these later documents only show a disposition towards delayed hominization and are not the Church making a declaration on the matter as the Congregation for the Doctrine of the Faith in its 1987 Instruction Donum vitae or, On Respect for Human Life in it’s Origin and on the Dignity of Procreation: Replies to Certain Questions of the Day says,

The Magisterium has not expressly committed itself to an affirmation of a philosophical nature [as to the time of ensoulment], but it constantly affirms the moral condemnation of any kind of procured abortion. This teaching has not been changed and is unchangeable. The human being is to be respected and treated as a person from the moment of conception, and therefore from that same moment his or her rights as a person must be recognized, among which in the first place is the inviolable right of every innocent human being to life. (I, n. 1).

Thus as a summary, the Theological principles say that we must believe that man has one rational or intellectual soul (and not two souls). That this rational or intellectual soul is the form of the human body, that it is immediately created by God, and not propagated by the soul of the father. That this intellectual soul is immortal and is multiplied by the number of bodies it is infused into and not shared by all men. A fetus does not begin to have a rational soul only at birth. Finally, it is permitted to hold to delayed rational ensoulment so long as one still holds that the human being is to be respected and treated with the dignity of a person from the moment of conception.

Objection 1: In his Constitution Apostolicae Sedis, Pope Bl. Pius IX excommunicates those who procure an abortion and makes no distinction between the animated [hominized] and unanimated fetus when previously there was a distinction. But if the church no longer makes this distinction then it doesn’t recognise this distinction. Therefore, the Church no longer recognises unhominized fetuses (i.e. it agrees with immediate ensoulment).

Response: I distinguish the major: The Church no longer making the distinction in penal law means it rejects the philosophical distinction between animated and unanimated fetuses, denied; the Church removes this distinction for pastoral and practical reasons while remaining neutral on the philosophical question, conceded. I similarly distinguish the minor: The Church doesn’t recognise the distinction for practical reasons, conceded; it doesn’t recognise it to settle the philosophical debate about when hominization occurs, denied. Therefore, I deny the conclusion.

First part is proved: When the Church modifies its penal law, this can be done either to define doctrine or for pastoral prudence. But the removal of the animation distinction in Apostolicae Sedis was done for pastoral prudence, since (a) the document concerns ecclesiastical penalties not doctrinal definitions, and (b) the probability of immediate hominization made the distinction pastorally imprudent. Therefore, the removal of this distinction does not constitute a doctrinal judgment on when hominization occurs.

Second part is proved: A disciplinary decree can remove practical distinctions while remaining neutral on theoretical questions. But Apostolicae Sedis is a disciplinary decree concerning ecclesiastical penalties. Therefore, its removal of the practical distinction between animated and unanimated fetuses does not settle the theoretical question of when hominization occurs.

Objection 2: In Dignitas Personae (2008) it says that 'The human embryo has, therefore, from the very beginning, the dignity proper to a person' [DH 5116]. But what has the dignity of a person is a person (i.e. has a rational soul). Therefore, the human embryo, from the very beginning, is a person.

Response: I distinguish the major: What has the dignity of a person must have a rational soul, denied; what has the dignity of a person either has a rational soul or a proximate disposition to receive a rational soul from God, conceded. I similarly distinguish the minor: The human embryo has the dignity of a person because it actually has a rational soul, denied; it has the dignity of a person because it has a proximate disposition to receive a rational soul from God, conceded. Therefore I deny the conclusion.

The distinction is proved in two ways:

First, from Church teaching: The Church attributes dignity not only to what actually possesses a rational soul but also to what is ordained or disposed to personhood. As taught in Ad resurgendum cum Christo, even the human body after death deserves dignity as "an integral part of the human person whose body forms part of their identity," though a corpse has only a remote potency to reunion with the soul through resurrection. Therefore, if a body deserves dignity through remote potency to receiving a soul, much more does an embryo deserve dignity through its proximate potency to becoming disposed (or proximate disposition) to receive a rational soul from God.

Second, from reason: That which has proximate potency to receiving its natural perfection deserves to be treated according to what it will become by nature. But the human embryo from conception has the proximate disposition to receive a rational soul from God through natural development. Therefore, the embryo deserves personal dignity from conception, even before receiving a rational soul.

Objection 3: Psalm 51:5 says, “Behold, I was brought forth in iniquity, and in sin did my mother conceive me.” But original sin requires a rational soul. Therefore, we have a rational soul from conception.

Response: I distinguish the major: "In sin did my mother conceive me" means I have original sin formally at conception, denied; it means I was conceived with the material disposition to original sin (fomes peccati), which would lead to the formal aspect of original sin upon receiving a rational soul, conceded. I similarly distinguish the minor: Original sin in its formal aspect requires a rational soul, conceded; the material disposition to original sin requires a rational soul, denied. Therefore, I deny the conclusion.

First part is proved: According to St. Thomas's own commentary on this passage, he understands it to refer primarily to the "tinder" of sin (fomes peccati) that remains even after parents are cleansed of original sin through circumcision. As he explains, "Baptism and circumcision cleanse the soul from original guilt, but the 'tinder' still remains, and circumcision happened in the flesh, and man generates fleshly sons according to the flesh." Therefore, the passage can be understood as referring to the material disposition to sin present from conception, without requiring the presence of a rational soul at that moment.

Second part is proved: The transmission of original sin occurs in stages: first, the body is generated with the fomes peccati; second, when the rational soul is infused, the person incurs the debt of original sin; and third, immediately after ensoulment, the person lacks sanctifying grace. This understanding is confirmed by St. Thomas when he notes that even children of baptized parents must be baptized, showing that the flesh carries the material disposition to sin independently of the formal aspect of original sin. Therefore, being "conceived in sin" refers to the beginning of this process (material aspect) without requiring the completion of the process (formal aspect) at the moment of conception.

Objection 4: Christ’s human nature is identical to ours in all essential respects. In Christ’s human nature, the rational soul was present from the very moment of conception. Therefore, in all human beings, the rational soul is present from the moment of conception.

Proof of major: The Council of Chalcedon defines that Christ in his humanity was “like unto us in all things but sin.” [DH 301] The Letter to the Hebrews also says this in two places, “Therefore he had to be made like his brethren in every respect” [2:17] and “For we have not a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sinning” [4:15].

Proof of the minor: St. Thomas agrees, and so do all Theologians.[61]

Response: From St. Thomas, “The form actually gives the species; but the matter in itself is in potentiality to the species. And hence it would be against the nature of a form to exist before the specific nature. And therefore the dissimilarity between our origin and Christ’s origin, inasmuch as we are conceived before being animated, and Christ’s flesh is not, is by reason of what precedes the perfection of the nature, viz. that we are conceived from the seed of man, and Christ is not. But a difference which would be with reference to the origin of the soul, would bespeak a diversity of nature.”[62]

Further, I distinguish the major: That Christ’s human nature is identical to ours in its essential characteristics, conceded; that Christ’s mode of conception and ensoulment was identical to ours in every aspect, denied.

I distinguish the minor: That Christ had His rational soul from the moment of conception, conceded; that this establishes a universal principle for human generation, denied. Therefore, I reject the conclusion.

Proof of the first part: The Council of Chalcedon affirms that Christ took on our nature, but it does not teach that His manner of conception was identical to ours. If this argument were valid, we would also have to argue that all human beings should be conceived of a Virgin or without original sin—which is absurd.

The proof of the second part was explained in the proof from St. Thomas.

5. Philosophical perspective on the debate.

Since Theology is neutral to the question of delayed hominization, it is necessary to investigate the philosophical aspects of this debate. To do this I will discuss several philosophical principles that will help us to understand the position of delayed hominization.

The first of the philosophical principles is that the intellective soul is the substantial form of the human body. This is proven by the theological principles we gathered from the Magisterium in the Council of Vienne and the Fifth Lateran Council.

The second of the philosophical principles is that matter must be proportionate to form. This will need to be defended.

The third philosophical principle closely follows the previous one and that is that the human soul can only exist in matter (or a subject) that is sufficiently disposed to receive it. The Magisterium approved this principle in the 15^th Thomistic Thesis, but since it wasn’t formally defined it will still need to be defended.

The fourth philosophical principle is that in the order of generation the imperfect precedes the perfect, and thus the substantial form (as in a composite) is the terminus, not the principle, of the generative process. This will also have to be defended.

The fifth philosophical principle is that nature does nothing in vain. This will also be defended.

Principle 2: Matter must be proportionate to form.

Proof from St. Thomas: “Now form and matter need always to be mutually proportionate and naturally adapted, so to speak, because the proper act is produced in its proper matter.”[63]

Proof from reason: Potency is perfected by Act, but what is perfected by another must be proportionate to it. Therefore, matter (as potency) must be proportionate to form (as act).

Proof of the minor: Proportionality is a relationship between things with respect to comparative degree. But perfection is a comparative degree in being. Therefore, what is perfected by another must be proportionate to it.

Principle 3: The human soul can only exist in matter that is sufficiently disposed to receive it.

Proof from the Church: The Magisterium approved the 15^th Thomistic Thesis which says, “On the other hand, the human soul subsists by itself. When it can be infused into a subject that is sufficiently disposed…”

Proof from St. Thomas: “The whole man comes from the thigh of the begetter—insofar as the power of the seed coming from the thigh conduces to the union of soul and body by giving the matter its ultimate disposition, which necessitates the form, by reason of which union a man is a man…”[64]

Proof from reason: Disposition is the quality of readiness to act. A sufficient readiness to act is necessary in matter for act to operate. Therefore a sufficient disposition is necessary for act (form) to operate.

Proof of the minor: Act can only operate through the matter in which it exists. But matter cannot support an operation without being ready for that operation. Therefore a sufficient readiness to act is necessary in matter for act to operate.

Principle 4: In the order of generation the imperfect precedes the perfect, and thus the substantial form (as in a composite) is the terminus, not the principle, of the generative process.

Proof from St. Thomas: “In all things produced by generation, nature proceeds from the imperfect to the perfect.”[65]

“He says that an agent does make a sphere to be, for he makes it from bronze, which is the matter, as the principle of generation, and from sphere, which is the form and terminus of generation.”[66]

“the intellectual power does not belong to a corporeal organ, as the power of seeing is the act of the eye; for understanding is an act which cannot be performed by a corporeal organ, like the act of seeing. But it exists in matter so far as the soul itself, to which this power belongs, is the form of the body, and the terminus of human generation.”[67]

Proof of the first part from reason: That which is generated moves from potency to act. But movement from potency to act proceeds from imperfect to perfect. Therefore, generation must proceed from imperfect to perfect.

Proof of the second part from reason: Generation proceeds from imperfect to perfect. But the substantial form gives perfect act to matter. Therefore, the substantial form (as in a composite) must be the terminus, not the principle of generation.

Principle 5: Nature does nothing in vain.

Proof from St Thomas: “Nature does nothing in vain”.[68]

Proof from reason: What acts for an end does nothing in vain. But nature acts for an end. Therefore, Nature does nothing in vain.

Proof of the major: If something acts for an end, its actions are purposeful by definition, and thus not "in vain" (i.e., not futile or purposeless).

Proof of the minor: What always or for the most part acts in the same way acts for an end. But nature always or for the most part acts in the same way. Therefore nature acts for an end.

These philosophical principles are commonly accepted among Thomists and even many other Catholic philosophical traditions. So, what is the main distinction between those who defend delayed hominization and those who defend immediate hominization?

The main difference seems to come from how to interpret the part of the definition of the soul which states ‘having life in potency’. Those who defend immediate hominization tend to support the view that this potency can be a remote potency, whereas those who support delayed hominization believe this potency is a proximate potency. Here is the distinction.

Having life in (proximate) potency. Since this part of the definition is referring to vital operations, these operations being in proximate potency means that some operation must be immediately possible. For a vegetative soul this would be the nutritive power, for the sensitive soul, this would be some vital sense operation like touch, for the rational soul this would be a vital rational operation like the abstracting power of the agent intellect. Each kind of soul must have some minimal operation it can immediately perform according to its nature.

Having life in (remote) potency. Operations in remote potency mean that the operations of the particular kind of soul must be at least in some way possible if natural generation continues while not needing to be immediately possible. So, an animal with a sensitive soul does not need to be able to operate any sensitive operation immediately, so long as there is a remote potency for them through natural generation.

The position of having life in proximate potency has been the most common in the Thomistic tradition and was in fact a main reason why many people supported immediate hominization when the theory of preformationism was popular, since, according to the theory, the embryo is already fully formed from the beginning of conception and so had the proximate potency to all its proper vital operations. However, the position of remote potency became popular from Cartesian dualism since in their philosophy the soul is not the substantial form of the human body but a separate substance, yet since this was rejected in Principle 1 as against defined theological principles, we must also reject Cartesian dualism.

I will show that the position of ‘having life in proximate potency’ is required if one accepts the previous five principles.

Argument 1: A body that cannot immediately actualise any of its proper vital operations is not sufficiently disposed to its soul. But a body must be sufficiently disposed to receive its soul. Therefore, a soul must be able to immediately operate at least some of its proper vital operations.

Proof of major: A particular soul is defined by its proper operations e.g. a vegetative soul is defined by the vital operations of nutrition, growth, reproduction; a sensitive soul by the vital operations of sensation; and the rational soul by its vital operations of intellect and will. But a soul that can’t immediately act on any of its proper vital operations can’t be that particular kind of soul as form is limited by matter.

The proof of the minor is found in our defence of principle 3.

Argument 2: A higher soul with none of its proper vital operations would be pointless if a lower soul could sufficiently operate its proper vital operations. But nature does nothing in vain. Therefore, a soul cannot be infused unless the body is sufficiently disposed to at least some operation beyond a lower soul’s capability.

Proof of major: If an entity only exhibits vegetative functions (e.g., nutrition), then a vegetative soul would be sufficient, i.e. a higher soul would be superfluous. But form must be proportionate to matter (principle 2). Therefore, a higher soul cannot be present in matter that can only support operations of a lower soul.

Proof of minor is from principle 5.

Objection 1: In the order of generation, the imperfect precedes the perfect. But the use of proper vital operations by a proper soul can still be under development without being immediately operable. Therefore, a proper soul doesn’t need to immediately operate its proper vital operations.

Proof of major is from principle 4.

Proof of minor is from St. Thomas, “the use of reason depends in a certain manner on the use of the sensitive powers; wherefore, while the senses are tired and the interior sensitive powers hampered, man has not the perfect use of reason, as we see in those who are asleep or delirious. Now the sensitive powers are situated in corporeal organs; and therefore, so long as the latter are hindered, the action of the former is of necessity hindered also; and likewise, consequently, the use of reason. Now children are hindered in the use of these powers on account of the humidity of the brain; wherefore they have perfect use neither of these powers nor of reason.”[69]

Response: I distinguish the minor: the use of some proper vital operations by a proper soul can still be under development without being immediately operable, conceded. The use of all proper vital operations, denied. I likewise distinguish the conclusion: a proper soul doesn’t need to immediately operate all its proper vital functions, conceded. Any of its proper vital functions, denied.

I freely grant that a proper soul doesn’t need to be able to immediately act on all of its proper vital operations. A plant may not have the power of generation until it has sufficiently developed, but still has a vegetative soul during this development. However, I argue that it at least needs to have some minimal vital operation proper to its proper soul (i.e. it still has the power of nutrition at least). Likewise, a fetus may have a rational soul and not be able to immediately act its vital operation of reasoning, but it must still have some minimal vital operation proper to a rational soul.

Therefore, if a body cannot immediately actualise any operation proper to its soul, it is not yet sufficiently disposed to receive that soul. This is supported by St. Thomas, “in the generation of an animal or a man in which the form is most perfect, there are many intermediate forms and generations, and consequently corruptions, since the generation of one is the corruption of another. Therefore, the vegetative soul, which comes first when the embryo lives the life of a plant, is corrupted, and is succeeded by a more perfect soul which is both nutritive and sensitive, and then the embryo lives an animal life. When this is corrupted, it is succeeded by the rational soul introduced from without”[70]

Objection 2: The succession of souls during embryonic development would require the embryo to undergo multiple substantial changes, transforming from one species to another (vegetative to sensitive to rational). But substantial changes involve the corruption of one being and the generation of another entirely new being. Therefore, if delayed hominization were true, the adult human would not be the same being that began at conception, violating the principle of identity and continuity of the human person.

Proof of major: The substantial form determines the species of a being. As St. Albert the Great notes, successive souls would mean the embryo is "changing from species to species." [71] Each substantial change would mean the previous being ceases to exist entirely and a new being comes into existence. This is not mere development but ontological replacement.

Proof of minor: The unity of human identity requires that the being who exists as an adult is the same being who came into existence at conception, not a succession of ontologically distinct beings. If delayed hominization were true, then the vegetative being that existed initially would have been destroyed and replaced by a sensitive being, which would later be destroyed and replaced by a rational being. This would mean that no human person has ever existed from conception—a position incompatible with the unity and continuity of personal identity.

Response: From St. Thomas, “Before it has the rational soul, the embryo is not a perfect being, but is on the way to perfection; and therefore, it is not in a genus or species save by reduction, just as the incomplete is reduced to the genus or species of the complete.”[72]

I distinguish the major: The succession of souls during embryonic development would require the embryo to undergo multiple substantial changes, transforming from one species to another if these souls were a perfect and complete soul, conceded. That these souls are perfect and complete, denied. I likewise distinguish the minor: Substantial changes of perfect beings involve the corruption of one being and the generation of another entirely new being, conceded. The substantial changes of imperfect transitory beings, denied. Therefore, I deny the conclusion.

Proof: In the generation and corruption of imperfect and transitory beings, the imperfect and transitory being that is corrupted is subsumed into the generation of the new being. Thus, the vegetative soul in the embryo is not completely corrupted but is subsumed into the sensitive soul so that the new substantial form is both a sensitive and vegetative soul as St Thomas said in what was quoted in the response to the previous objection. Thus, the identity of the human from conception to adulthood is maintained by the continuation of the subsumed vegetative grade which explains why many point to the DNA in the nucleus as a sign of continuity throughout different stages of man.

Having supported this theory of ‘having life in proximate potency’ being the proper understanding of the definition of a soul we can now move on to figuring out what are the minimal vital operations of each proper soul.

St. Thomas gives us the answer for the vegetative soul, “the generative power is served by the augmentative and nutritive powers; and the augmentative power by the nutritive.”[73] Therefore, the nutritive power is the most foundational.

Regarding the sensitive soul, touch is the most foundational.[74] About this St. Thomas says, “But although there are several sensory powers, still there is one which is the basis of the others, namely touch, in which every sensible nature is principally rooted.”[75]

Lastly, regarding the rational soul, St. Thomas says, “The intellectual soul is indeed actually immaterial, but it is in potentiality to determinate species. On the contrary, phantasms are actual images of certain species, but are immaterial in potentiality. Wherefore nothing prevents one and the same soul, inasmuch as it is actually immaterial, having one power by which it makes things actually immaterial, by abstraction from the conditions of individual matter: which power is called the agent intellect; and another power, receptive of such species, which is called the potential intellect by reason of its being in potentiality to such species.”[76] However, what cannot function without another is dependent on it, and thus the latter is more foundational.[77] Therefore, the agent intellect is the most foundational operation of the rational soul. About this St. Thomas says, “But now the agent intellect is not an object, rather is it that whereby the objects are made to be in act: for which, besides the presence of the agent intellect, we require the presence of phantasms, the good disposition of the sensitive powers, and practice in this sort of operation; since through one thing understood, other things come to be understood, as from terms are made propositions, and from first principles, conclusions.”[78]

Therefore, we can say that the agent intellect depends on phantasms (sensible species) in the imagination for its operation which if possible in a body would constitute a sufficiently disposed body for the rational soul. St. Thomas confirms this doctrine when he says, “Therefore, if the human soul is capable of being united to a body, because it needs to receive intelligible species from things through the intermediary of the senses, then the body, to which the rational soul is united, must be one which can most adequately present to the intellect those sensible species from which are derived the intelligible species existing in the intellect. Hence the body to which the rational soul is united must be best disposed for sensory operation.”[79]

Objection 3: Many adult humans are not immediately able to operate their rational vital operations or even their sensory vital operations due to injury, disease or some other reason, such as coma or severe neurological impairment. However, they still have a rational soul. Therefore it is not necessary that someone be able to immediately operate some minimal rational vital operation to have a rational soul; rather, having life with remote potency is sufficient.

Response: I distinguish the major premise: Many adult humans are not immediately able to operate their rational or sensory vital operations due to injury or disease, conceded; that this implies they lack a sufficiently disposed body for the rational soul under normal conditions of generation, denied. I similarly distinguish the minor premise: They still have a rational soul because they retain the substantial form of a human being despite accidental impediments, conceded; because a rational soul can be present without any immediate capacity for its proper operations, denied. Therefore, I deny the conclusion that remote potency alone is sufficient for the presence of the rational soul.

Proof: The key distinction lies in the difference between accidental and substantial impediments, which determines whether a body is sufficiently disposed to receive and retain the rational soul. An adult who cannot immediately operate rational vital operations due to injury (e.g., brain trauma), disease (e.g., Alzheimer’s), or other conditions (e.g., coma) suffers from an accidental impediment. This means the body’s organic structures were once sufficiently developed during generation to support the rational soul’s operations but have been accidentally altered or impaired.

In contrast, an embryo in its early stages of generation lacks this sufficient disposition substantially to immediately perform rational vital operations. The objection conflates two contexts: the persistence of the rational soul in an already-formed human versus its initial infusion during generation. In the adult, proximate potency was once actualized whereas in an embryo, proximate potency was never actualised.

Further: The objection’s claim that remote potency suffices overlooks this generative requirement. If remote potency were enough, the rational soul could be infused at conception, yet be incapable of any operation until weeks or months later which as we have already argued would be superfluous and contrary to nature’s order (Principle 5). Instead, St. Thomas’s epigenesis ensures each stage has a form proportionate to its matter (Principle 2), with the rational soul entering only when the body can support its distinctive acts. Thus, the adult’s case reinforces proximate potency as a condition for the rational soul’s presence, not remote potency.

It seems clear that if the imagination is not sufficiently formed to produce phantasms at the moment of conception, then our philosophical principles support delayed hominization.

6. The perspective of the empirical sciences.[80]

In this part, we will look at what observations the empirical sciences can tell us about embryonic development and how these observations can be interpreted by our philosophical principles. To do this I will break down the embryonic development from fertilization up until birth in the stages that are currently used in embryology. Afterwards, I will give an explanation of how this can be interpreted philosophically and bring up certain phenomena and how the Thomistic position would explain it. Before we begin, I will briefly note that science continues to change and the observations and conclusions in science today may be outdated tomorrow. Hence, I am using the latest observations and conclusions by well-respected embryologists. If embryology changes then the conclusions we form from applying philosophy to embryology also need to change. I only note this because embryology may develop even further in the future and the dates I give for certain developments may shift, thus my philosophical conclusions will also have to shift. Also, those who support delayed hominization and immediate hominization don’t disagree on the empirical data, but only on how to interpret it.

Germinal Stage (Weeks 1-2)

1. Fertilization: Conception occurs when a sperm cell fertilizes an egg cell in the fallopian tube, resulting in a zygote—a single-cell entity containing a complete set of chromosomes.

2. Cleavage: The zygote undergoes rapid mitotic divisions, known as cleavage, as it travels down the fallopian tube toward the uterus. These divisions increase the number of cells without increasing the overall size of the zygote.

3. Morula Formation: Approximately three days post-fertilization, the zygote transforms into a morula—a solid ball of cells resembling a mulberry.

4. Blastocyst Formation: By the fifth day, fluid accumulates within the morula, leading to the formation of a blastocyst. This structure comprises an inner cell mass, destined to develop into the embryo, and an outer cell layer called the trophoblast, which will contribute to the placenta.

5. Implantation: Around six to ten days after fertilization, the blastocyst reaches the uterus and embeds itself into the uterine lining, a process known as implantation. Successful implantation is crucial for establishing nutrient and oxygen exchange between the mother and the developing embryo.

Throughout the germinal stage, the developing organism relies on nutrients stored in the egg and secretions from the fallopian tubes and uterus.

Embryonic Stage (Weeks 3-8)

1. Gastrulation (Weeks 3-4):

• Formation of Germ Layers: The embryo undergoes gastrulation, resulting in the establishment of three primary germ layers:

• Ectoderm: Develops into the nervous system, skin, and hair.

• Mesoderm: Forms muscles, bones, the circulatory system, and internal structures.

• Endoderm: Gives rise to the lining of the digestive and respiratory tracts, as well as associated organs.

2. Neurulation (Weeks 3-4):

1. Neural Tube Formation: The ectoderm folds to create the neural tube, which will develop into the brain and spinal cord. This process is typically completed by the end of the fourth week.

3. Organogenesis (Weeks 4-8):

1. Heart Development: The heart begins as a simple tube and starts beating around day 22-23, establishing the foundation for the circulatory system.

2. Limb Formation: Limb buds emerge, which will lengthen and differentiate into arms and legs.

3. Facial Features and Sensory Organs: Structures such as the eyes, ears, and nose start to form, contributing to the development of facial features.

4. Internal Organ Development: The rudimentary forms of major organs, including the liver, kidneys, lungs, and digestive tract, begin to take shape and organize into systems.

4. Morphological Changes:

1. Somite Formation: Blocks of mesoderm called somites form along the neural tube, giving rise to the vertebral column, skeletal muscles, and dermis.

2. Body Folding: The flat embryonic disc undergoes folding, resulting in the establishment of the body plan with a defined head, tail, and lateral body folds.

By the end of the embryonic stage, the embryo has developed the basic structures of a human body, and all major organ systems have begun to form.

Fetal Stage (Weeks 9-Birth)

Weeks 9 to 12

Growth: The fetus grows to approximately 6–7.5 cm in length and weighs about 14 grams by the end of week 12.

Physical Development:

Facial Features: The face becomes more defined, with the formation of the nose and lips, giving the fetus a more human-like appearance.

Limbs: Arms and legs are fully formed, and the fetus can make spontaneous movements, though these are not yet perceptible to the mother.

Genitalia: External genitalia begin to differentiate, but distinguishing sex via ultrasound is typically not possible until 13-14 weeks.

Other Developments: Fingernails start to develop, and the kidneys begin producing urine, which is released into the amniotic fluid, contributing to its volume.

Sensory Development:

Touch: Early reflexes emerge, such as moving away from stimuli, indicating the beginnings of touch sensitivity tied to the developing nervous system and skin.

Smell: Olfactory receptors begin to develop around week 11.

Cognitive Development:

The brain divides into its basic regions—forebrain, midbrain, and hindbrain. Neurons begin forming and migrating, laying the groundwork for future cognitive functions.

Weeks 13 to 16

Growth: The fetus reaches about 15 cm in length.

Physical Development:

Skin and Hair: A fine layer of hair called lanugo covers the body, providing some insulation, while the skin remains thin and translucent.

Skeletal System: Bones continue to ossify, becoming harder and more structured.

Movements: The fetus becomes more active, performing movements such as sucking and swallowing, which are precursors to feeding after birth.

Other Developments: The liver and pancreas begin producing secretions, marking the onset of digestive system functionality.

Sensory Development:

Taste: Taste buds become functional (weeks 13–15), enabling the fetus to taste the amniotic fluid, which reflects the mother’s diet.

Cognitive Development:

The cerebral cortex starts to develop, though it remains rudimentary, setting the stage for higher cognitive processes later.

Weeks 17 to 20

Growth: The fetus measures approximately 20 cm in length.

Physical Development:

Skin and Hair: Eyebrows and eyelashes form, and hair on the head becomes more apparent, adding to the fetus’s distinct features.

Movements: Fetal movements, known as "quickening," may be felt by the mother, typically between 18 and 20 weeks, signaling increased activity.

Other Developments: The fetus begins to develop a regular sleep-wake cycle, indicating early behavioral patterns.

Sensory Development:

Touch: The nervous system matures, allowing the fetus to respond to external stimuli, such as pressure or movement within the womb.

Vision: The eyes can move, though they remain closed, as the visual system continues to form.

Smell: Olfactory receptors may begin to function around week 20.

Cognitive Development:

The brain contains billions of neurons, and synapses begin forming rapidly, establishing the neural connections necessary for future learning and memory.

Weeks 21 to 24

Growth: The fetus reaches about 28 cm in length and weighs approximately 500 grams.

Physical Development:

Lung Development: The lungs produce surfactant, a substance essential for breathing after birth, though they are not yet fully functional.

Skin: The skin is wrinkled and translucent, with fat deposition beginning to provide insulation and energy reserves.

Reflexes: The fetus exhibits reflexes such as grasping and startle responses, indicating nervous system development.

Other Developments: Fingerprints and footprints continue to develop, becoming more defined and unique.

Sensory Development:

Hearing: The inner ear is fully developed by week 18, and by week 24, the fetus can hear and respond to external sounds, such as the mother’s voice or loud noises, with movements or heart rate changes.

Cognitive Development:

Basic memory and learning emerge through habituation, where the fetus stops responding to repeated stimuli, reflecting an early capacity to adapt to its environment.

Weeks 25 to 28

Growth: The fetus measures approximately 35 cm in length and weighs about 1000 grams by 28 weeks.

Physical Development:

Brain Development: Rapid brain growth occurs, increasing its complexity and surface area to support future cognitive functions.

Eyes: The eyelids open, and the fetus can detect changes in light and dark, even through the mother’s abdominal wall.

Lungs: The lungs continue to develop, improving the chances of survival if born prematurely, though they remain immature.

Other Developments: The nervous system matures, enhancing regulation of body functions like heart rate and movement.

Sensory Development:

Vision: The eyes open and close, and the fetus responds to light, marking the onset of visual perception.

Hearing: Hearing becomes more refined, with the fetus recognizing familiar sounds, such as the mother’s voice, suggesting early auditory memory.

Cognitive Development:

The brain’s surface begins to fold (gyrification), increasing its capacity. Recognition of familiar sounds indicates the development of short-term memory.

Weeks 29 to 32

Growth: The fetus reaches about 38–43 cm in length and weighs approximately 1.5 kg.

Physical Development:

Fat Accumulation: Increased body fat smooths out the skin’s appearance, aiding in temperature regulation after birth.

Bone Development: Bones are fully developed but remain soft and flexible to facilitate passage through the birth canal.

Movements: The fetus exhibits more coordinated movements and can respond to external sounds with kicks or shifts.

Other Developments: The fetus stores essential minerals like iron, calcium, and phosphorus for postnatal growth.

Sensory Development:

All five senses—touch, taste, smell, hearing, and vision—are functional, with continued refinement preparing the fetus for the outside world.

Cognitive Development:

Ongoing brain maturation supports improved regulation of physiological functions, such as breathing patterns, laying the foundation for postnatal cognitive growth.

Weeks 33 to 36

Growth: The fetus measures approximately 40–48 cm in length and weighs between 2.5 to 3 kg.

Physical Development:

Lanugo: The fine hair covering the body begins to disappear, remaining only on the upper arms and shoulders.

Nails: Fingernails reach the tips of the fingers and may extend beyond, indicating growth completion.

Body Fat: Continued fat accumulation enhances temperature regulation and provides energy reserves for after birth.

Other Developments: The fetus typically assumes a head-down position in preparation for delivery.

Sensory Development:

Sensory systems are well-developed, fully capable of processing input.

Cognitive Development:

The brain continues to mature, supporting the transition to more advanced functions postnatally, though complex cognition like reasoning remains undeveloped.

Weeks 37 to 40

Growth: Considered full-term, the fetus typically measures 48–53 cm in length and weighs between 3 to 3.6 kg.

Physical Development:

Organ Maturation: All organ systems are fully developed and functional, ready for independent life outside the womb.

Skin: The skin is smooth, and the vernix caseosa (a protective waxy coating) begins to decrease, though some may remain at birth.

Hair: Head hair is now coarse and thicker, varying in amount and texture among individuals.

Other Developments: The fetus practices breathing movements, and the lungs are fully mature, capable of efficient gas exchange upon birth.

Conclusion

It is clear that nutrition begins at fertilisation first from nutrients inside the egg and then later after implantation externally through the uterine wall. The beginning of growth depends if you consider cleavage as growth or whether the increase in physical size counts. The first cleavage usually happens around 30 hours after fertilisation. Physical growth happens during the blastocyst stage after it loses the zona pellucida at around 5 days after fertilisation. Generation doesn’t properly happen until gametogenesis begins, but Benedict Ashley O.P. argues that monozygotic twinning could be considered a form of asexual generation which can happen from the first cleavage stage typically up until implantation with some very rare cases happening after implantation up until day 14-15. I don’t find this as a plausible explanation philosophically but we will address monozygotic twinning later.

Touch is the first sense that begins to develop and different studies suggest different starting points. The sense of touch begins to develop at around 8 weeks, but may not be fully functional until 14 weeks. Taste functioning around 13-15 weeks. Smell functioning around 20 weeks. Hearing begins around week 18 with the fetus responding to sounds by week 24. Sight begins later with the eyes opening at around 28 weeks.

The interior senses of the common sense, imagination, memory, and cogitative powers are more difficult to assess since they depend on some sufficient development in the brain and assessing when they begin is very speculative and not precise due to the current limitations in neuroscience. Evidence from vibro-acoustic stimulation suggests that the fetus has developed the ability to integrate different senses like touch and hearing between weeks 20-24 and this would correspond to the development of the common sense faculty. However, it is difficult to know if this is just reflexive or perceptual. Since the imagination receives its impressions from the common sense it would probably function around the same time as the common sense power at 20-24 weeks. Habituation to repeated stimuli has been observed as early as 24 weeks, with consistent habituation being seen at 28 weeks this would correspond to the development of memory. The cogitative power may develop as early as 28 weeks when associative learning is first evidenced, but may begin developing later at 30-36 weeks when melody memory is first evidenced or it might even be post-natal.

This sequence of development seems to support delayed hominization. Firstly, because it clearly shows that preformationism is wrong and supports epigenesis. Secondly, because it aligns with the progression from vegetative faculties like nutrition and growth to sensation with the vegetative stage not requiring sensation, but rather sensation requiring the vegetative functions. Lastly, the interior sense powers required for minimal rational operations (i.e. the agent intellect) are sufficiently developed some time after sensation begins but before birth and thus the body is not sufficiently disposed to receive the rational soul until some time after basic sensation has already begun. Thus we come to my argument:

Argument 1: The sufficiently disposed matter of the body for the rational soul to be infused requires the production of phantasms by the imagination. But the production of phantasms is entirely impossible at the moment of conception. Therefore, the rational soul is not infused at the moment of conception (i.e. delayed hominization is true).

Proof of the major: This comes from the requirements for the minimal vital operation of the rational soul, being the agent intellect. The agent intellect requires sensible species in the phantasm to abstract them into intelligible species that can be used by the possible intellect. For the body to be sufficiently disposed to the rational soul (principle 3), phantasms must be at least proximately possible, if not actual in the imagination.

Proof of the minor: Phantasms come from sense input from the common sense power, and these depend on sense input from the external senses. Embryological data clearly shows that no senses are formed before the 8^th week and functionality of these senses may not occur until the 13^th week. It is at least certain that no sense data exists in the embryo before the 8^th week and thus, no phantasms can exist before this point. Furthermore, the neurological structures needed to combine this sense data into the common sense faculty require sufficient differentiation of the brain which doesn’t occur before the 9^th week. While the exact timing of hominization can’t be more precise until we have better neurological data and better advances in neuroscience we can put hard outer limits on when it can’t happen before, and must have happened after. It certainly can’t happen before the senses are developed which is at the 8^th week. It must have happened before habituation occurs which requires the processing of sense data and at least some phantasm by the 28^th week.

There are other phenomena that also support delayed hominization.

The first is monozygotic twinning. Meth. M. Hudeczek O.P. gives a strong account of why this phenomenon supports delayed hominization.[81] His arguments are thus:

Argument 2: Embryonic matter from first cleavage up to at least the stage where twinning is impossible is indifferent and undetermined as to whether it is the principle of part of an individual or of the whole organism [or “is totipotent” in modern terminology]. But this indifference and indetermination regarding being whole or being part manifests only the presence of a common and inferior vital principle, excluding the superior principle, which is the rational soul. Therefore, the rational soul cannot be present in the first stages of prenatal development.

Proof of the major is evident from the scientific evidence [of totipotent cells].

Proof of the minor: First part: the embryonic matter manifests the presence of an inferior vital principle, and this is evident, because the fetus already from the beginning of its conception has some life. But a living thing lives at least by a vegetative principle, which is the lowest vital principle common to all corporeal living things.

Second Part: that it is "only" this principle, "excluding the superior one"; for a special soul [soul of a particular species] requires a special determination actually existing. But the aforementioned indifference manifests the absence of a special soul.

Argument 3: A divisible vital principle cannot be the rational soul. But according to Didymology [study of twins], the vital principle in the embryonic state is divisible. Therefore, according to Didymology, the rational soul cannot be present in the embryonic state.

Proof of the major: The rational soul, being spiritual, is simple (indivisible).

Proof of the minor: A vital principle which informs the body in an extended mode is divisible. But according to Didymology, the soul informs the human body in the first stages of embryonic development in an extended mode. Therefore the soul of this state is divisible.

Proof of the last major: To inform in an extended mode is the same as to be intrinsically affected by quantity.

Proof of the last minor: A soul which does not regard the whole organization as necessary for itself to inform, informs the body in an extended mode; while one which regards the whole organization as necessary for itself to inform, informs the body in an unextended and indivisible mode. But the soul of the human body in the first states of embryonic evolution does not regard the whole organization as necessary for itself to inform. Therefore it informs the body in an extended mode.

Proof of the last major is evident.

Proof of the last minor: If the whole living being is divided into parts, and the parts after division manifest true life and are restored to a complete organism, this is a sign that not the whole organization of the whole is necessary for informing. But this is found from Didymology regarding the human embryo. Therefore...

Argument 4: Indeed if animation occurred in the first instant of conception, the animation of twins would have to be understood either that in the primitive ovum there were multiple rational souls which would successively, upon separation, inform the parts, or that individual souls would be successively created and infused on the occasion of the separation of any part of the germ. But both disjuncts are inconvenient. Therefore, in the first embryonic stages the rational soul is not present.

Proof of the major: The major is a complete disjunction; for the human soul is excluded because it is spiritual.

Proof of the minor: The first is inconvenient because multiple substantial forms cannot coexist in one and the same body simultaneously. Moreover, the disposition to divide itself is indifferently present in every germ whether division is actually carried out or not; the execution of division occurs due to external acting causes alone. Hence if one wishes to explain the fact of division through plurality of souls, one must recur to God's intention to procreate from this germ rather than from another multiple individuals. But then it is asked: and does this happen without respect to secondary causes? Why, I ask, does God do this in this germ and not in another, anticipating the creation of souls? And why in one marriage is the birth of twins repeated, while in others it never occurs?

The second is inconvenient for two reasons: a) because with different embryonic states the actual disposition of matter also varies and therefore the form, which per se requires the ultimate disposition of the subject in an indivisible way, would be capable of informing diversely disposed matter and therefore would be indifferent regarding informing this matter rather than another; which is certainly contrary both to the nature of form and to the principle of information. And if it is responded that this matter is always disposed from the beginning to receive the human soul because from it nothing but a human can be generated, this must be carefully accepted for the reasons explained above… For the matter of the ovum is disposed only in potency to receiving the human soul. But that which actually exists manifests only a certain common life. Indeed the process of development is directed, as to an end, to forming a human individual; but it does not necessarily indicate the presence of the rational soul, for according to St. Thomas, the formative power already placed in the seed suffices for such developmental direction, which doctrine reappears in the Biology of our time.

Benedict Ashley O.P. objects to the third argument that these two aren’t the only options. He argues that splitting can be analogous to asexual reproduction, budding, or cloning.[82] He argues that at fertilisation there is a single rational soul, and then after the splitting, the new organism is immediately infused by a new rational soul. However, Ashley doesn’t explain how this reconciles with how this division can be possible in a spiritual soul which Hudeczek explains isn’t possible in his second argument. To me, Hudeczek’s arguments still stand unrefuted except that a clarification should be made: a soul itself isn’t divisible, but is potentially multiple from a division that preserves a sufficient organisation for that proper soul. It also seems that Ashley would have to concede that if splitting is analogous to asexual reproduction then the zygote can’t have a proper human nature, since it is proper to human nature to generate through the production of gametes. Asexual reproduction is more proper to lower lifeforms, not humans, and so something that can generate in a way analogous to asexual reproduction would properly have a vegetative soul, not a properly human (rational) one.

There are other objections that are raised based on the interpretation of the embryological data that we will address here.

Objection 1: Any organism that exhibits intrinsically directed, continuous development toward a specific nature must possess the substantial form corresponding to that nature throughout its development. The human embryo, from fertilisation onward, exhibits intrinsically directed, continuous development toward a specifically human nature. Therefore, the human embryo possesses the substantial form specific to human nature (the rational soul) from fertilisation onwards.

Response: I distinguish the major: Any organism that exhibits intrinsically directed, continuous development toward a specific nature must possess the substantial form corresponding to that nature throughout its development, denied. Must possess a formative power with the potential for a nature through its development, conceded. I distinguish the minor: The human embryo, from fertilisation onward, exhibits intrinsically directed, continuous development toward a specifically human nature as rational, denied. As sufficiently disposed to the reception of a rational soul from God, conceded. I, therefore, deny the conclusion.

Proof: The substantial form is not the principle but the terminus of generation as was defended in our fourth philosophical principle. Moreover, the formative power doesn’t require the substantial form of the nature it is developmentally directed to, but only that the nature be in the formative power potentially. One explanation for this can come from the idea that the nucleus has the genetic material to develop an organism into a human with sense operations while the organism itself doesn’t have any sense operation. Just as these sense operations exist in the genetic material as a potential for the organism, it still requires an active power to develop this potential into an actuality. The same thing can be said of the development of a human as a whole which is not sufficiently disposed to receive a rational soul from fertilisation, but becomes sufficiently disposed later.

Objection 2[83]: The zygote's development must be explained either by (1) the presence of the human substantial form from fertilization, or (2) an instrumental power that causes subsequent substantial changes. The instrumental power explanation is metaphysically uneconomical and lacks empirical verification. Therefore, the zygote must possess the human substantial form (rational soul) from the moment fertilization is complete.

Proof of the major: The zygote's nucleus contains the complete genetic blueprint for development and the active power to transform itself into a mature human being. This genetic continuity and self-directed development must be explained either by the presence of the substantial form from the beginning or by positing an instrumental power that causes substantial changes at later stages.

Proof of the minor: An instrumental power explanation requires positing:

1. Occult powers with no direct empirical confirmation

2. Multiple substantial changes without observable discontinuities

3. A complex causal mechanism where simpler explanation suffices

4. A violation of epigenetic continuity between developmental stages

Response: I distinguish the major premise: The zygote's development must be explained either by (1) the presence of the human substantial form from fertilization, or (2) an instrumental power that causes subsequent substantial changes, denied as an incomplete disjunction. The zygote's development must be explained by either (1), (2), or (3) a formative power as an intrinsic efficient principle in the zygote that progressively disposes matter to receive the rational soul at a later stage, conceded. Therefore, I bypass the minor, and deny the conclusion.

Proof: The formative power (virtus formativa) in Thomistic philosophy is not an instrumental power, but an intrinsic efficient principle as St. Thomas says.[84] The distinction is important as an instrumental power does not contain the power to produce its own effect but must be elevated by an agent to produce an effect beyond its own nature. Whereas an intrinsic efficient principle does have the power to produce its effects at least virtually. It seems necessary to explain the formative power in more depth to show why this objection doesn’t work.

The formative power according to St. Thomas, is the power that first forms the body through generation, and then continues in the body to regulate it through nutrition after generation.[85] The formative power is related to the matter of a thing in the same way that the form of a house in the mind of a builder is related to the bricks and timber.[86] Therefore, we can say that the formative power is an intrinsic efficient principle in the vegetative soul which contains the form of the complete material organism virtually, and acts as the efficient cause to generate this form and then sustain it through nutrition. So does this formative power fail to meet any of the four challenges in the objection’s proof of the minor premise?

This power isn’t occult without any direct empirical confirmation because it nearly perfectly matches up with the role of the nucleus, or the developmental system in the developmental systems theory.

It bypasses the problem of multiple substantial changes without observable evidence because substantial changes can just refer to the different grades of souls that develop from the formative power (i.e. the vegetative soul to the sensitive soul).

While this power does depend on complex causal mechanisms (especially if one holds to developmental systems theory), simpler explanations (like the idea that the substantial form is driving generation) usually fail for their own reasons as we have already copiously explained.

Finally, it doesn’t violate epigenetic continuity between developmental stages as it isn’t an instrumental cause and the formative power contains the complete material form virtually.

Objection 3: The brain is not able to support conceptual thought until some months after birth which would be necessary for rational vital operations. However, hominization can’t occur after birth (due to the condemnation of Innocent XI). Therefore, “having life in proximate potency” is false and thus delayed hominization is false too.

Response: I distinguish the major: The brain cannot support actual exercise of conceptual thought until some months after birth, conceded; the brain cannot support the agent intellect's minimal operations until after birth, denied. I similarly distinguish the minor: Hominization cannot occur after birth, as rightly condemned by Innocent XI, conceded; hominization requires the immediate exercise of conceptual thought, denied. Therefore, I deny the conclusion.

Proof: As we already stated, rational faculties can develop gradually after the rational soul is infused because the sufficient disposition to receive the rational soul is not the full exercise of all the rational faculties, but that the proper soul’s minimal operations are exercisable. This is tied to the development of the imagination to produce phantasms for the agent intellect to abstract. St. Thomas in multiple places says that children especially but even some adults don’t and possibly will never develop the use of reason, but still acknowledges that they have a rational soul.[87] Therefore, the full use and exercise of reason and conceptual thinking is not required for the infusion of the rational soul but only the minimal operation (i.e. the use of the agent intellect).

Objection 4: An embryo from conception is nourished in the way animals do, not in the way plants do from inorganic matter. But what is capable of vital and animal operations is capable of receiving a rational soul. Therefore, from conception the embryo has a rational soul.

Response: I distinguish the major: An embryo from conception performs the vital operation of nutrition in the manner of animals, I bypass. That the operation of nutrition is a properly sensitive operation, denied. I likewise distinguish the minor: What is capable of sufficient vegetative and sensitive operations is capable of receiving a rational soul, conceded. That an embryo from conception is capable of such operations, denied. Therefore, I deny the conclusion.

Proof of the first part: I bypass the proposition because it isn’t clear that nutrition is significantly distinguished between all plants and animals. One example is carnivorous plants like the Venus fly trap which receives nutrients from organic matter. Since this point isn’t important for my response I bypass the topic. However, even if such a distinction did exist, the actual manner of operation doesn’t make the nutritive power somehow become a sensitive power. The diversity among the ways that organisms exercise the nutritive power doesn’t change the fact that the power belongs to the vegetative soul properly.

Proof of the second part: Embryos don’t have the sufficient sensitive functions to receive a rational soul from conception, namely the sufficient use of the imagination to produce phantasms as has been proven in argument 1.

Objection 5: If in the beginning, only a vegetative soul were present, there could be no cause of the sensitive soul, since we see that nothing in the plant kingdom ever develops into an animal. But the succession of souls posits that such development takes place. Therefore, the succession of souls is wrong.

Response: I distinguish the major: that a vegetative soul can’t cause itself to become a sensitive soul in perfect and complete beings, conceded. That a vegetative soul in which the formative power with a virtual material form of an animal couldn’t effectively cause the virtual material form already within the vegetative soul to make it a sensitive soul, denied. I likewise distinguish the minor: that the succession of souls posits that a vegetative soul can cause itself to become a sensitive soul in perfect and complete beings, denied. That this can happen through the formative power of incomplete and transitory beings, conceded. Therefore, I deny the conclusion.

Proof of the first part: We have already discussed that the formative power can produce through generation what it contains virtually as a form. The cause doesn’t come from the vegetative soul qua vegetative soul, but according to the potential form in the formative power in a transitory vegetative soul. The formative power causes a sensitive soul, because it contains the sensitive soul within its virtual material form and acts on that virtual material form in the wider organism. This can be seen from how the DNA virtually contains a complete human body and the developmental system acts on the DNA to grow the organism according to this blueprint from conception to full human maturation passing from mere vegetative operations to sensitive operations.

6. Further objections

Moral Implications of Delayed Hominization

Objection 1: Delayed hominization says that an embryo isn’t properly a human person until later in development. However, homicide is defined as killing a human person. Therefore, delayed hominization says that killing an embryo before hominization isn’t murder (an act of homicide).

Response: I distinguish the major: Delayed hominization holds that an embryo lacks a rational soul (and thus full personhood) until later in development, conceded; that it denies the embryo should be treated as a person with inviolable dignity from conception, denied. I distinguish the minor: Homicide is strictly defined as killing only a being with a rational soul, denied; it extends to killing a human life with proximate potency to personhood, conceded. Therefore, I deny the conclusion

Proof: As argued under “Theological Perspective” (Objection 2), the Church teaches that the embryo must be respected as a person from conception, not because it has a rational soul immediately, but because it possesses a proximate disposition to receive one. This disposition arises from the formative power, an intrinsic efficient cause, directing the embryo’s development from a vegetative soul toward a body sufficiently organized for the infusion of the rational soul. At fertilization, the embryo’s unique DNA and active potency mark it as human in potency, reducible to the human species in its incomplete and transitory state, akin to how a seed is of its plant’s species. Killing it violates this natural ordination, an act grave enough to constitute murder, as it destroys a life ordered to its perfection. Homicide, in moral theology, need not require an actual person but encompasses unjustly ending human life so disposed. Thus, the pre-hominized embryo’s death is murder despite its delayed ensoulment.

Further: The objection assumes personhood’s absence negates moral status, but we can distinguish a metaphysical status from a moral status. The embryo’s dignity and protection stem from its proximate disposition, not its current state. This shows that delayed hominization is compatible with a fully Catholic and pro-life position.

Objection 2: Only someone with a rational soul can be the subject of Baptism. But embryos before hominization do not have a rational soul. Therefore, embryos before hominization can’t be validly baptised.

Response: I concede both the major and minor and thus the conclusion. This issue has a long history in the church with historical church rulings making a distinction between hominized and non-hominized (animated and non-animated) fetuses. However, conditional baptism in cases of doubt is still the prudential pastoral practise precisely because while delayed hominization may be far more probable, it isn’t certain and immediate hominization is still somewhat probable thus causing some doubt and allowing for conditional baptisms in cases of emergencies. The Code of Canon Law in Canon 871 thus says, “If aborted fetuses are alive, they are to be baptized insofar as possible.”

Other Objections

Objection 3: Delayed hominization is ambiguous and complex about when hominization begins, whereas immediate hominization is very clear and simple. But a simpler explanation is usually more true. Therefore, immediate hominization is true.

Response: I distinguish the major: Delayed hominization is more complex in requiring developmental thresholds for rational ensoulment, conceded; it is more complex in explaining the entire phenomenon of human development, denied. I likewise distinguish the minor: A simpler explanation is usually more true when it adequately accounts for all relevant data, conceded; when it ignores or contradicts empirical and philosophical difficulties, denied. Therefore, I deny the conclusion.

Proof: Delayed hominization only appears more ambiguous and complex because it relies on scientific and empirical data which is not precise and can seem complex. However, its philosophical principles are very clear and precise and bypass certain difficulties that still remain for immediate hominization such as how a spiritual soul can be divided from twin splitting, why a superfluous soul that can’t operate any of its proper operations would be infused at conception rather than at the time it can operate them, how a rational soul can be proportionate to the matter of a zygote since matter limits form, etc. When the entire picture of prenatal development is considered, delayed hominization is the simpler explanation.

Further, Ockham’s razor says that entities should not be multiplied beyond necessity. However, the apparent complexities in delayed hominization are necessary to explain the philosophical principles and the empirical data. Remember that many of the great thinkers of the Church still held to delayed ensoulment despite the apparent complexity.

7. Conclusion

In conclusion, the Thomistic defence of delayed hominization rests on the hierarchical tripod of theology, philosophy, and the empirical sciences. The Church’s neutrality permits it, our five philosophical principles show why it is a strong position, and the empirical data showing progression from vegetative operations to sensitive operations show why this position is far from being outdated. Phenomena like monozygotic twinning further show the problems that still remain for immediate hominization. As I said at the beginning, this is not a speculation on what St. Thomas Aquinas would have thought today if he had our current empirical data, only that using the Thomistic framework we can show how delayed hominization is a very probable, and rational position based on contemporary embryological data. It is also compatible with the moral teachings of the Catholic Church regarding the sanctity of life and the dignity that ought to be afforded to human life from the moment of conception.

However, we can’t remain complacent as this debate will surely continue and scientific advances will force us to shift our understanding. We should always thank God for giving us such a brilliant teacher as St. Thomas, and strive in every way to imitate his splendid virtues. St. Thomas Aquinas, prince of doctors, prince of theologians, the common and angelic doctor of the most Holy Catholic Church, pray for us.

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[1] https://www.catholic.com/qa/was-st-thomas-aquinas-wrong-about-when-human-life-begins

[2] https://www.thecatholicthing.org/2022/05/18/aquinas-on-when-human-life-begins/

[3] Maryniarczyk, Andrzej, 'The Dispute over Delayed Animation: When does a Human Being Begin?', Studia Gilsoniana (2020), 9, pp. 423-65 <10.26385/SG.090317>.

[4] https://www.catholic.com/magazine/online-edition/a-person-from-the-moment-of-conception

[5] Pascendi Dominici Gregis para. 45.

[6] Joseph F. Donceel S.J., 'Immediate Animation and Delayed Hominization', Theological studies (Baltimore) (1970), 31, pp. 76-105 <10.1177/004056397003100103>.

[7] Benedict Ashley O.P., The Ashley Reader: Redeeming Reason, (Naples, FL: Sapientia Press, 2006), p. 329-68.

[8] Much of this history is based on Joseph F. Donceel, 'Immediate Animation and Delayed Hominization'; Hyacinthus-M. Hering O.P. ‘De tempore animationis foetus humani’, Angelicum (1951), 28, pp. 18-29, https://www.jstor.org/stable/44623148; and, Murillo-González, Jorge, 'Evolution of embryology: A synthesis of classical, experimental, and molecular perspectives', Clinical anatomy (New York, N.Y.) (2001), 14, pp. 158-63 <10.1002/1098-2353(200103)14:2<158::AID-CA1025>3.0.CO;2-Q>.

[9] Aristotle, The Complete Works of Aristotle, ed. by Jonathan Barnes, (Princeton, NJ: Princeton University, 1984) 736a27-b28.

[10] On the Making of Man, 28 (PG 44, 230).

[11] Ad Amphilochium, ep. 2 (PG 138, 587).

[12] Ambiguorum liber de variis difficilibus locis Sanctorum Dionysii Areopagitae et Gregorii Theologi (PG 91, 1335).

[13] Graecarum affectionum curatio: Sermo de natura hominis (PG 83, 942).

[14] De ecclesiasticis dogmatibus 14 (PL 58, 984).

[15] Ep. 121, 4 (CSEL 56, 16).

[16] Quaestionum in Heptateuchum libri 7: Quaest. De Exodo 80 (CSEL 28/2, 148).

[17] This is what Edouard Hugon says in his Philosophia Naturalis II P., treatise 2, q. 4, a. 2, n. 4; but St. Albert the Great could also be seen as defending delayed hominization in his Treatise on Animals, bk. 16, tr. 1, ch. 11.

[18] Liber de conceptu virginali et originali peccato 7 (PL 158, 440).

[19] Sententiarum libri quatuor 2, d. 18, 8 (PL 175, 418)

[20] Explanatio in canticum b. Mariae (PL 175, 418).

[21] St. Albert the Great’s position is disputed but it seems more probable that he held to delayed hominization according to ‘L’animazione dell’embrione in Alberto Magno e Tommaso d’Aquino (I-II)’ by José A. Izquierdo Labeaga in Alpha Omega, 22, n.2, 2019, pp. 275-315.

[22] Sentences 2, d. 18, q.2, a. 3 ad 4; Summa Contra Gentiles 2, c. 68, c. 86-89; Compendium Theologiae 1, cc. 92-95; De Potentia q. 3, aa. 9-12; Q. disp. De anima q. 8-12 (but especially q. 11 ad 1); Summa Theologiae Ia, q. 118, aa. 1-3.

[23] According to Ashley O.P. loc. cit., p. 345-47.

[24] Purgatorio, Canto 25, vv. 52-62, 67-75.

[25] This is clear from St Albert the Great’s refutation of Plato’s preformationism in his Problemata Determinata, J.A. Weisheipl (ed.), Aschendorff, Münster 1975, p. 61, “This was Plato’s error, who believed that the semen was a small animal and that the semen of man was a little human (homunculus).”

[26] Theologia Moralis 3, tract. 4, no. 394; 6, tract. 2, no. 124.

[27] Psychol. Intellect. P. III, lect. 3.

[28] Inst. Metaph. spec., III, Psych., pars II, thesis XII, Parisiis, 1904, p. 157.

[29] Praelectiones Philosophicae, 9th ed., Parisiis, 1909, n. 272.

[30] Commentaire français littéral de la Somme théologique, Toulouse, 1910, 5, p. 650.

[31] Compendium Philosophiae Scholasticae, II, n. 553 (Romae, 1912).

[32] Principia Philosophica, Florentiae, 1914, II, n. 374.

[33] Psychologie, Louvain, 1923, 10th ed., p. 337.

[34] Cursus Philosophiae Scholasticae II, Nat. Philos., Bilbao, 1924, n. 425-428.

[35] Summa Philosophiae Scholasticae V, Psychol., 5th ed., Rome, 1925, n. 244.

[36] Les grandes theses de la philosophie thomiste Paris, 1928, p. 205.

[37] Manuale Theol. Mor., II, n. 138.

[38] Les points fondementaux de la philosophie thomiste, Turin, p. 163-164.

[39] Philos. Schol., 54th ed., Paris, 1932, I, n. 237.

[40] Theologia Moralis, II, n. 622; Epitome Iuris Canonici, II, n. 31.

[41] Quaestiones de Embryologia, Liege, 1927, p. 67-68.

[42] Summa Philosophiae, Taurinis, 1936, II, 882-883.

[43] Circulus Philosophiae IV, Psychologia, Taurinis, 1938, n. 788; cf. nn. 797, 960.

[44] Elementa Philosophiae, Parisiis, 1937, II, p. 542-543.

[45] Traité de Philosophie, II Psychologie, Lyon, 1940, p. 676

[46] La questione del momento in cui l’anima razionale è infusa nel corpo, Rome, 1940.

[47] Evolution and Theology: The Problem of Man's Origin, London, 1934, p. 87.

[48] L'origine de l'âme humaine, Québec, 1945, p. 143.

[49] Thomistic Philosophy: Four volumes in one, Philosophy of Nature, n. 477, Charlottetown, CA, 1950, p. 271.

[50] Estudios de Psicologia experimental, Madrid, 1948, vol. II, pp. 519-569.

[51] 'Immediate Animation and Delayed Hominization', Theological studies (Baltimore) (1970), 31, pp. 76-105 <10.1177/004056397003100103>.

[52] William A. Wallace, St. Thomas on the Beginning and Ending of Human Life. https://www3.nd.edu/~maritain/jmc/ti/wallace3.htm

[53] Aristotle, De Anima (On the Soul), 412a20-21.

[54] For more on the definition of a soul see Edouard Hugon’s Philosophia Naturalis: Biologia et Psychologia, First treatise, q. 2, a. 3 [translation forthcoming by myself].

[55] Emphasis mine.

[56] Emphasis mine.

[57] Donceel S.J. in loc. cit. p. 89n47 found many editions from 1662 to 1923 which all have this wording.

[58] Bullarum...amplissima collectio 5/1 [Rome, 1751] 26a.

[59] Ibid., p. 275b.

[60] Collectanea de prop. fide 1 (Rome, 1907) no. 282, p. 92.

[61] Summa Theologiae IIIa, q. 6, a. 4.

[62] Summa Theologiae IIIa, q. 6, a. 4, ad 2.

[63] Summa Contra Gentiles 2.81.

[64] De Potentia, q. 3, a. 9 ad 2.

[65] Summa Theologiae Ia, q. 101, a. 2.

[66] Comm. On Metaphysics 7, lect. 7 n. 1424 (emphasis mine). See n. 1421 for the fact that form is not the terminus by itself, but the form as it comes to be in something else (namely the composite). In this sense, we say that the substantial form is the partial terminus quo, but the physical composite is the total terminus quo.

[67] Summa Theologiae Ia, q. 76, a. 1, ad 1.

[68] Comm. on De Caelo bk 1, lect. 13, n. 132; Comm. on De Anima bk 3, lect. 11, n. 851; Summa Contra Gentiles 2.55, 3.48.

[69] Summa Theologiae Ia, q. 101, a. 2.

[70] Summa Contra Gentiles 2.89

[71] De Animalia 16, tr. 1, c. 16.

[72] De Potentia, q. 3, a. 9, ad 10.

[73] Summa Theologiae Ia, q. 78, a. 2.

[74] Summa Theologiae Ia, q. 78, a. 3 (especially ad 3 and 4).

[75] Disp. Q, De Anima, q. 8.

[76] Summa Theologiae Ia, q. 79, a. 4, ad 4.

[77] See Comm. on Metaphysics, bk. 5, lect. 13

[78] Summa Theologiae Ia, q. 79, a. 4, ad 3.

[79] Disp. Q, De Anima, q. 8.

[80] I am relying on these works for the empirical sciences on embryology: T. V. N. Persaud, and Mark G. Torchia, The developing human: clinically oriented embryology, (Philadelphia: Elsevier, 2025); and, T. W. Sadler, and Jan Langman, Langman's medical embryology, (Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012).

[81] Meth M. Hudeczek, 'De tempore animationis foetus humani secundum Embryologiam hodiernam', Angelicum (1952), 29, pp. 162-81<http://www.jstor.org/stable/44624144>.

[82] Benedict Ashley O.P., The Ashley Reader: Redeeming Reason, p. 366.

[83] From Benedict Ashley O.P., The Ashley Reader: Redeeming Reason, pp. 361-63.

[84] Comm. On Metaphysics, bk. 7, lect. 8, n. 1442. Many seem to think that the virtus formativa is an instrumental principle, but St. Thomas is clear here.

[85] De Potentia, q. 3, a. 9, ad 16. Many mistakenly think that the virtus formativa is lost after generation but St. Thomas is very clear here.

[86] Comm. On Metaphysics, bk. 7, lect. 8, n. 1451.

[87] Summa Theologiae IIIa, q. 68, a. 12 (especially ad 2).

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