Jacques Maritain Center: Thomistic Institute

St. Thomas on the Beginning and Ending of Human Life

William A. Wallace, O.P.

[This article is reprinted from an essay of the same title that appeared in Autori Vari, Sanctus Thomas de Aquino Doctor Hodiernae Humanitatis. Studi Tomistici 58 (Vatican City: Libreria Editrice Vaticana, 1995), pp. 394-407. Page numbers have been inserted in brackets for the reader who might wish to cite the original pagination.]


St. Thomas is well known for his teaching that the beginning of human life is a gradual process, that the human soul is not infused into the incipient organism at fertilization but rather is prepared for by a succession of forms that dispose the matter for the reception of a rational soul. Less well known is his speculation that the reverse process may occur at the ending of human life, namely, that the human soul may depart from the body before all signs of life have disappeared from it. There are sensible signs that processes like these might occur, for example, in the stages of "quickening" mothers feel when pregnant and in the gradual loss of faculties experienced by moribund individuals in the period before their death. Yet, until fairly recently, there was little medical information available that would lend support to such thinking. Thus it is commonly taught that human life in the most proper sense begins at fertilization, when the rational soul is infused into the body, and terminates at death, when the human soul departs from the body.

In this essay I sketch a few developments in medical science that encourage us to look again at St.Thomas's teaching on the beginning and ending of human life, not to make the point that his doctrines are perennially valid, but rather to stimulate our development of a natural philosophy along Aristotelian-Thomistic lines that is adequate to deal with problems now arising in embryology and neuroscience. Unfortunately, in recent times the philosophy of nature has languished within Thomism and has largely been replaced by a philosophy of science that is incapable of dealing with distinctively human problems, particularly those relating to the soul and its relation to the body. Natural philosophy is an integral part [395] of the Thomistic synthesis and it would be a serious mistake to let it now fall by the wayside. Its neglect can only place in peril the more fashionable metaphysical themes that continue to attract attention in the present day.

St. Thomas's Teaching

With regard to human generation, Aquinas followed Aristotle in holding that the conception of a male child was not completed until the fortieth day after intercourse, whereas that of the female child lasted until the ninetieth day. The only instance of immediate conception he cites is that of Jesus Christ, on the basis that Christ's conception, unlike that of other humans, should not have to await the complete formation of his natural flesh. On this account Christ was conceived instantly by the divine power of the Holy Spirit, and thus in miraculous fashion (In 3 Sent., d.3, q.5, a.2). For all others conception is a gradual process. Animation, of course, is immediate in the sense that a soul of some type is present as soon as the male's semen fertilizes the material provided by the female, but this is not a human soul at the outset. In its earliest stage it is a nutritive soul (anima vegetativa), which regulates the early growth of the embryo; when development is sufficient to support sensation, the nutritive soul is replaced by a sense soul (anima sensitiva); and this in turn is ultimately replaced by the human soul (ST 1a, q.118, a.1, ad 4). In this teaching Aquinas is simply following Aristotle's account in On the Generation of Animals (729a-744b). Aristotle recognizes that it is difficult to determine precisely when the rational soul comes to be present in the embryo and simply states that it comes from outside and is divine (G.A. 736b). St. Thomas picks up on this teaching and explicitly holds that the intellective soul (anima intellectiva) is created by God and infused into the embryo at the completion of the human's coming-to-be, and that this soul once present performs all the functions of previous forms in the incipient organism (ST 1a, q.118, a.2, ad 2). So, for him, this complete conception of the male child does not occur until after forty days, and that of the female until after ninety days, as just mentioned.(1)

[396] An interesting feature of Aristotle's teaching on the earlier stages of animal generation is the role he assigns to the nutritive soul and the sense soul in the developmental process. In the case of the former, he assumes that the semen and the unfertilized embryo, while still separated from each other, already possess a nutritive soul, although they do so only potentially. Such an embryo thus lives the life of a plant, first with the mother drawing nourishment to it and then with the embryo beginning to nourish itself as a whole; when this second stage occurs the nutritive soul loses its potential status and comes to be present actually (G.A., 736b). Similarly, at the onset of animal life the sense soul is present only potentially. For it to become actually present sense organs have to develop in the organism, and particularly the sense of touch, so that the embryo can experience sensation. Since there can be various degrees of sensation, moreover, and in the higher animals all of the sense organs have to be developed before a specific animal is produced, Aristotle held that the developing animal embryo first becomes an animal in general and then one of a particular type. The transition process can then occupy a considerable period of time. That serves to explain why the sensitive soul in the developing human, the animal with the most refined sense powers, requires at least forty days to reach complete actuality.

St. Thomas apparently subscribed to this aspect of Aristotle's teaching and in so doing introduced the concept of a transient entity, an ens in via, into the discussion. The text in which he does so occurs in SCG 2, c.89, where Aquinas is discussing the intermediate souls or substantial forms that function in the generation of higher animals and humans. The successive replacement of forms in these cases takes place by a series of natural generations and corruptions, and thus, when leading up to the human soul, the form which is most perfect (forma perfectissima), there will be found many intermediate forms and generations. These intermediates, St. Thomas writes, are not complete in species but are on the way (in via) to a determinate species, and thus they are not generated with a permanent status but only transiently, so that, through them, the ultimate species may be arrived at.(2)

[397] Although St.Thomas is frequently cited for his advocacy of "delayed hominization," as it is now commonly called, little attention has been paid to the obverse of that process, what might be termed "early dehominization," where the human soul departs from the body at some period of time before all bodily functions have ceased. Aquinas gives indication that he endorsed this view of the ending of human life, although he does not seem to have developed his thought on that subject. The text in which he does so is his exposition of Proposition 1 of the Liber de causis, where he compares what happens in the generation of a human being with what happens at corruption. He writes: "For it is obvious that in the generation of an individual human being one finds in the material subject first existence, then the living thing and after that a human; for it is an animal before it is a man, as is said in the second book of On the Generation of Animals. And again, in the process of corruption, first [the individual] loses the use of reason and remains alive and breathing, then it loses life and remains a being, because it does not corrupt into nothingness."(3) Although this statement occurs in a commentary where St. Thomas is exposing the thought of the author of De causis, it seems that here he is speaking in his own voice. He clearly understands Aristotle's teaching on delayed hominization, and applies the reverse process, without objection, to suggest at least the possibility of early dehominization.


All of this discussion, it goes without saying, took place in a thought context where nothing was known about cells or of the existence of ova and [398] spermatazoa, where the microscope was more than three hundred years off, and molecular biology yet another three hundred years or so after that. Only within the past few decades, however, have substantial advances been made in embryology, and so now we stand at the apex of a development taking place over many centuries that awaits translation into the philosophical language of Aristotle and Aquinas. One way of effecting this translation is to focus on the concept of nature, first as instantiated in human nature, and then in the various natural kinds found in the world of nature--animals, plants, minerals, and the elements from which all of these are constituted. Aristotle defined nature as a principle within bodies that initiates their characteristic activities and reactivities, and identified it with the basic matter of which they are composed, but more properly with the form that energizes that matter and stabilizes these bodies as substances (Physics, 193ab). This latter was called substantial form within the Aristotelian tradition, but for purposes here it is better referred to as natural form, because it is by it that a thing's nature is known. Natural forms are immediately intelligible: if we know what a cat is, or what an oak is, or what sulphur is, and are able to define these, it is because we have grasped their natures and have come to know them through their natural forms.(4)

The natures of objects of ordinary experience seem relatively unproblematic, but natures prove difficult to grasp when we move into the realm of the very small. Atoms and molecules went undetected for centuries, and the vast number of subatomic particles that have recently been detected defy attempts at clear understanding. One may speak of the nature of water or iron, but does it make sense to attribute a nature to an electron, or a neutron, or a proton? Although these are entities to which we assign physical existence, they do not seem to be intelligible as wholes or subsistent entities, but rather as parts that enter into the composition of other things. Moreover, most subatomic particles are either charged or radioactive: if the first, they can be quickly attracted to, and absorbed into, the being of another entity; if the second, while being what they are they are already breaking down into something else. Having granted them physical existence, one would be hesitant to exclude them entirely from the world of nature, but at the same time it would not seem necessary to attribute to them stable natures such as are found [399] in the macroscopic objects of ordinary experience. To the extent that they have natures, these are perhaps best thought of as transient natures, the type found in transient entities or entia vialia, beings only "on the way" or becoming, not existing or subsisting in their own right.

Pursuant to this line of reasoning, the findings of modern science would indicate that transient natures play a more important part in the universe than hitherto expected, certainly far beyond the role attributed them by St. Thomas. According to the "Big Bang" theory of its origin, evidences of which are still discernible at the edges of our expanding universe, subatomic particles were the first entities to appear and have continued to play a key role in the evolution of stars and galaxies. Immediately after creation there was an extremely brief period of elementary particle activity at very high energies; then came a longer period for the formation of chemical elements and compounds, from which stars and planets were formed some five billion years ago; then periods of biogenesis during which were produced, at least on the planet Earth, the plants and animals inhabiting its surface; and finally the period of hominization, the culmination of biogenesis, when homo sapiens was created by God and the human race made its first appearance.

To translate this sequence into the language of natures, and also to incorporate both creation and evolution within it, we propose to indicate a natural form by the letters NF, then add to it subscripts to designate different types of natures-- "t" for transient, "i" for inorganic, "p" for plant, "a" for animal, and "h" for human, and to represent a creative act with a double-shafted arrow and an evolutionary process with a single-shafted arrow. The sequence described above then may be written as follows:


0 >> NFt > NFi > NFp > NFa >> NFh

The idea is that from nothing (ex nihilo), indicated by the zero (0) at the far left, God created the primordial matter that exploded into a universe of elementary particles, here designated as entities with transient natures (NFt). From these, over time and still under the causal action of the First Agent, the chemical substances we know as elements and compounds were educed from the potency of protomatter, all of these possessing stable inorganic natures (NFi). Then, by a steady process of evolution under the divine causality, when proper conditions were realized first plant natures (NFp) and then animal natures (NFa) emerged into being, as selected regions of the universe came to be populated with the higher forms of flora and fauna. Finally, at the last stage, when [400] all was ready for the most perfect form to appear, a new creative initiative on God's part was required. This is shown by the second arrow with a double shaft, here indicating the direct creation of the human soul (NFh). The natural processes of evolution may be sufficient to bring organisms to a level just below that of thought and volition, but of themselves they cannot progress to the final stage. God himself must complete the process, producing ex nihilo the human soul, tailored to match the ultimate disposition of matter as this has been prepared, over billion of years, for its reception. And this creative act, according to Catholic teaching, would be repeated each time throughout the centuries that a new human person came into existence, with its matter being likewise disposed, through the procreative action of the human parents, to receive an individual, incommunicable, and immortal soul.

According to this model, transient natures were involved in the formation of the universe, cosmogenesis, as it is currently understood. It remains now to see how such natures may be involved in the generation of organisms. One way of doing this is to update St. Thomas's medieval view of plant generation and express it in a notation similar to that used in formula (1). For this we continue to designate a plant nature by NFp, but now add additional subscripts to indicate the parent plant and its offspring as well as the intermediate nature that effects the transition between the two. The new subscripts are the following: "pP" refers to the parent plant, "pO" to the offspring, numerically different from the parent but pertaining to the same species, and "pT" to the transient plant-like nature of the seed during its development. Then we have:


NFpP > NFpT > NFpO

The agency involved in the first transition, from NFpP to NFpT, is a natural agency associated with the powers of a plant nature. Chemical materials are absorbed by the parent organism through its powers of nutrition, growth, and reproduction to form the genetic materials contained in the seed. Then, after separation from the parent, a form of life persists in the seed. This begins its own internal development through the incipient plant-like form, NFpT, which St. Thomas referred to as an "active force" (vis activa) that gives it a plant nature "according to first act" (secundum actum primum) though it is not yet a form in the full sense (ST 1a, q.118, a.1, ad 3 & ad 4). This suffices to draw nourishment to the incipient organism and direct its growth until its quantitative parts are sufficiently articulated to sustain a stable, individual plant nature of the species. At that point a new plant form, NFpO, is educed from the [401] potentiality of matter, and a new individual of the species has been produced.(5)

The generation of a human individual is obviously more complex, but for St. Thomas it proceeds along analogous lines. Here again we would update his medieval account by importing into it information from modern embrylogical research, the details of which cannot be explained in this brief account.(6) We employ the same notation as heretofore, using subscripts similar to those in formula (2), with the subscript "hP" now referring to the human parent at the beginning of the process and the subscript "hO" to the human offspring at the end. In this case two transient natures are required, one plant-like, represented by the subscript "pT," the other animal-like, represented by the subscript "aT." The formula for human generation may then be written as follows:


2NFhP > nNFpT > mNFaT >> 1NFhO

This formula is similar to (2) but is definitely more complex. As before, the subscript "hP" refers to the human parents at the beginning of the process and the subscript "hO" to the human offspring at the end. Since human generation is bisexual, the process starts with two mature human souls or natures (indicated by the "2" preceding the NFhP), one female and the other male, and it terminates in one human offspring (indicated by the "1" preceding the NFhO), since we are not now considering the [401] production of twins. The double-shaft on the last arrow shows that the soul of the offspring is created directly by God, although the organic materials suitable for the soul's reception have been procreated by the parents.

The intermediate stages are symbolized by the two intermediate terms representing two transient natures, one plant-like, designated by subscript "pT," the other animal-like, represented by subscript "aT." But note here that a small "n" and a small "m" have been prefixed to the two symbols so as to number these types of genetic material. In the normal case and in our modern understanding, the process starts with two seeds (n=2), an egg and a sperm cell, although there might be more than one sperm if multiple births were being discussed. They combine to form a unicellular zygote, but that cell quickly divides and subdivides to form a complex cell mass that grows and nourishes in a way analogous to plant life--hence the subscript "pT." In later stages of development this embryonic human develops organs of sensation and movement and so manifests the characteristics of animal life--hence the subscript "aT." Note again the "m" before the term NFaT. Recent embryological studies have shown that the number "m" is not necessarily the same as the number "n," whatever that might be, nor is it necessarily the "1" that precedes the individual human offspring, 1NFhO. The initial cell mass is apparently made up of pluripotential cells that are not predetermined as to the number of organisms it will eventually produce. Twinning can take place during its development and, what is even more unusual, recombination can take place where splitting had previously occurred. Thus it is not always the case that n=2 and m=1, as one might expect. What starts out as apparently one organism might end up as two or more, and what starts out as two or more might end up as one.

When can one be sure that twinning or recombination will no longer occur and that the developing embryo has become irreversibly individual? Empirical evidence suggests that this occurs at the beginning of the third week after fertilization, when the "primitive streak" first appears in the embryo. Through this streak the cells of the embryo first become organized "into one whole multicellular individual living human being, possessing for the first time a body axis and bilateral symmetry" (Ford, p. 172). The appearance of one primitive streak thus signals that only one embryo proper (and thus a human individual) has been formed and begun to exist. At this time that the embryo "becomes one living body, informed or actuated by a human form, life-principle or soul that arises through the creative power of God" (ibid.).

This extraordinary finding provides strong confirmation of St. Tho[403]mas's view that transient natures continue to inform the incipient human organism for some time after the initial formation of the zygote. If God had created the human soul and infused it into the zygote at fertilization, then a stable individual of human nature would already have been formed. And, were another individual to be formed subsequent to that time, this would be an instance of asexual generation--a type of generation found in lower forms but not proper to humans. The phenomena of twinning and recombination therefore give unexpected support to St.Thomas's teaching that unstable natures, that is, transient natures, continue to inform the developing organism until such time as the proper dispositions are at hand for the irreversible formation of an individual human being. Only at that moment does God create a new soul, a substantial form that has a transcendental order to the matter so prepared for it, and infuse this intellectual soul into such matter, with the result that a new human person finally comes into being.


St. Thomas's suggestion that the human soul might depart from the body at some time previous to the cessation of various life signs, such as heart-beat in his day and brain death in our own, has not received as much consideration as his views on hominization. Two recent developments, however, bear on the account of dehominization he gives in his commentary on the Liber de causis and encourage us to give a closer look at the ending of human life. The first is a proposal from a neurophysiologist, Dr. Alan Shewmon, who has been studying brain death, persistent vegetative state, and dementia and speculating about the relevance of Thomistic concepts to their solution.(7) The other is an analysis of human death and dying provided by Mieczysaw A. Krpiec, O.P., a philosopher who, along with Karol Wojtya (Pope John Paul II), pioneered the movement known as Lublin Thomism and from its phenomenological perspective has been doing research in anthropology.(8)

Shewmon takes his point of departure from hypothetical brain-vat experiments wherein a human body is reduced to a brain alone, floating in a warm solution and connected to various machines that replace its [404] normal body functions. This type of thought experiment is used by Shewmon to inquire into the minimum part of the human body that is capable of supporting the human essence. The conclusion he comes to is that the brain is the critical structure for sustaining the human soul and mediating consciousness. If it were removed from the body by a skillful neurosurgeon, the organism's condition would be that of a brainless vegetative substance, the same as that of a person who has suffered total brain death. If still connected to life support systems, moreover, even though the cerebral cortex had been removed, as long as the brainstem was left intact the body need not die but could still be maintained in a vegetative state. On the other hand, if such systems were disconnected from the floating brain, the brain would die and with it the person, since the spiritual soul would leave the body at brain death.(9)

The case of dementia is more difficult to analyze, for it requires determining not only when irreversible damage had been done to the cerebral cortex but also pinpointing the specific part of the cortex that is necessary for the functioning of the human intellect and will. This part, which Shewmon sees as also essential for the functioning of the cogitative sense (vis cogitativa), he locates in the "tertiary association area" of the cortex. When this part is destroyed the human person dies and the rational soul departs, leaving only a humanoid animal body behind.(10)

Shewmon does not make explicit use of the concept of transient natures in his analysis of dehominization, but he does invoke a Thomistic concept that is closely related to it, namely, that of virtual presence. In his view, as in St.Thomas's, the spiritual soul contains virtually all the powers of animal and vegetative souls, and thus these souls may be said to be virtually present in the person. Should the right material dispositions be present, these souls can become actualized. For example, structural damage to critical parts of brain may be so severe that it forces a substantial change and results in the death of the person. Then the human soul leaves the body and a new soul is educed from the potency of matter: in severe instances of dementia this will be a sense soul, in instances of irreversible persistent vegetative states, a nutritive soul alone.

[405] Shewmon's analysis may now be summed up in formula (4), which turns out to be almost the obverse of our previous formulas:


NFh > NFaT > NFpT > NFi

Human cogitation is so dependent on the brain that, if the areas of the brain that are used by the cogitative sense for its operations are impaired or removed, they will cease to be informed by the human soul, which will be replaced by an animal soul virtually contained within the human. Expressed in the language of natures, in such a case human nature, NFh, degenerates into a transient animal-like nature, NFaT, and the individual shows the symptoms of dementia. If further deterioration takes place, such that the cerebral cortex no longer functions and only the brainstem remains intact, the transient animal-like nature gives way to a transient plant-like nature, NFpT, and the individual passes into the persistent vegetative state. And finally, when life signs cease altogether, not even this transient nature can be sustained in the organism and the body corrupts completely into inorganic matter, NFi, the "dust of the earth" from which it originally came.

Possibly because of his use of the "brain-vat" scenario, Shewmon may be viewed as writing science fiction and his thought dismissed as far-fetched speculation. This need not be the case, however, particularly when his ideas are taken up in the philosophical context provided by Father Krpiec. In discussing the death of a human person Krpiec makes a distinction between physical death, the cessation of life signs when the soul is thought to leave the body, which he calls "physical death," and death "understood actively," that is, death as a real experience of the human spirit.(11) The latter experience occurs at the moment when the person becomes capable of making a final decision about life, a moment that represents the culmination of all the changeable acts performed during the entire span of bodily existence. Active death, Krpiec argues, is a transtemporal experience that takes place in the realm of the spirit and beyond the point at which the individual can return to the temporal and changeable condition of earthly life.(12) Thus, it does not coincide with the co-activity of the body. The implication is that the human soul at the moment of active death has already departed from the body and subsists as an individual substance.

If one grants the absence of brain activity in the personal experience of death, one may draw a further corollary. According to St. Thomas's [406] theory of knowledge, all human knowing in the state of union with the body occurs by reflection on phantasms (ST 1a, q.84, a.7), which are produced by the cogitative power through the intermediary of various brain states. As long as the soul operates with phantasms, it can make changes through its higher powers of intellect and will, and it does not reach the point of ultimate decision. Conversely, at the moment in time when phantasmal activity ceases, these changes are no longer possible and the individual's rational life is over. If the intellect and will function later, they do so as separated substances and not as operative powers of a natural body. In other words, the person's truly human and changeable existence is ended, and the human soul, precisely as human, ceases to have any proper function it can exercise in the body.

If this analysis is correct, and the body continues to manifest vital activities, it probably does so as a humanoid organism. The body is specifically human, and thus should be classified under the human species, but it no longer possesses a stable human nature and will gradually decline and decay. Its life functions in this state can be seen as those of a transient nature, human in origin but sensory and vegetative in actual operation. There is therefore a succession of substantial forms in the humanoid organism, and the overall dying process can aptly be referred to as one of dehominization.

This surprising conclusion based on the work of Shewmon and Krpiec reinforces our earlier reasoning about transient natures based on the work of Ford, Pastrana, and others. When the two strains of thought relating to hominization and dehominization are put together, they constitute a strong counter-argument to those who would maintain that Thomism is dead, that it is a fossilized, archaic body of teaching, of antiquarian interest only, of no possible application in our miracle age of science and technology. Quite the opposite is true, and particularly in fields of knowledge relating to a Thomistic philosophy of nature and of science. Problems of greatest importance for human life are now awaiting solution: what to do at the beginning of life--contraception, "choice," in vitro fertilization, frozen embryos, surrogate motherhood, abortion directly or indirectly procured; what to do at the end of life--dementia, persistent vegetative state, "when to pull the plugs," a question increasingly being asked in hospitals. St. Thomas obviously has much to contribute to the solution of such problems. But there is another side of the coin. The philosophy of nature and of science has been much neglected in the recent Thomistic tradition. This is a difficult field of study, one in which much work needs to be done, particularly in applying hylomorphic concepts to natural processes at the micro-level, for example, [407] carrying the discussion of the "dispositions of matter" beyond the point it had reached in the thirteenth century. Problems relating to natures and transient natures have been broached in this essay, as have those relating to individuation and cogitation, but no claim can be made that the answers given are definitive.(13) New empirical data are constantly being made available, and unfortunately they are much more readily accessed than is a philosophical tradition that has been developing over seven centuries. Yet there is still much to be harvested in that tradition. Perhaps the stimulus provided by new discoveries in the physical and biological sciences will serve as a catalyst to our uncovering more of the riches that are there contained.


William A. Wallace, O.P., Ph.D., S.T.D. (Fribourg) is Professor Emeritus of Philosophy and History at the Catholic University of America, Washington, D.C., and Adjunct Professor of Philosophy at the University of Maryland, College Park, Md. He served as Director General of the Leonine Commission from 1976 to 1987. He has published fifteen books and over three hundred scholarly articles, many of which are concerned with the philosophy of modern science and the history of science from Albert the Great and Aquinas to Galileo, including specialized studies on Dietrich von Freiberg and Domingo de Soto. Since retiring from full-time teaching at Catholic University in 1988 at the age of seventy he has devoted himself almost exclusively to research and writing.

1. St. Thomas goes into further detail on the process of human generation in four places: In 2 Sent., d.18, q.2, aa.1,3; Summa contra gentiles 2, cc. 86-89; De potentia, q.3, aa.9-12; and ST 1a, q.118. All of these texts are analyzed by Michael A. Taylor in his "Human Generation in the Thought of Thomas Aquinas: A Case Study on the Role of Biological Fact in Theological Science" (S.T.D. diss., The Catholic University of America, 1981). A more extensive analysis that relates Thomas's teaching to studies in modern embryology is Norman M. Ford, When did I begin? Conception of the human individual in history, philosophy and science (Cambridge: Cambridge University Press, 1989), pp. 19-64.

2. The Latin here reads as follows: "Nec est inconveniens si aliquid intermediorum generatur et statim postmodum interrumpitur: quia intermedia non habent speciem completam, sed sunt in via ad speciem; et ideo non generantur ut permaneant, sed ut per ea ad ultimum generatum perveniatur." In a following paragraph Aquinas spells out in more detail what he means by these intermediates: "Quanto igitur aliqua forma est nobilior et magis distans a forma elementi, tanto oportet esse plures formas intermedias, quibus gradatim ad formam ultimam veniatur, et per consequens plures generationes medias. Et ideo in generatione animalis et hominis in quibus est forma perfectissima, sunt plurimae formae et generationes intermediae, et per consequens corruptiones, quia generation unius est corruptio alterius. Anima igitur vegetabilis, quae primo inest, cum embryo vivit vita plantae, corrumpitur, et succedit anima perfectior, quae est nutritiva et sensitiva simul, et tunc embryo vivit vita animalis; hac autem corrupta, succedit animal rationalis ab extrinseco immisa, licet praecedentes fuerint virtute seminis."

3. "Manifestum est autem in generatione unius particularis hominis quod in materiali subiecto primo invenitur esse, deinde invenitur vivum, postmodum autem est homo; prius enim ipse est animal quam homo, ut dicitur in secundo De generatione animalium. Rursumque in via corruptionis primo amittit usum rationis et remanet vivum et spirans, secundo amittit <vitam> et remanet ipsum ens, quia non corrumpitur in nihilum"--Super librum De causis expositio, ed. H. D. Saffrey (Fribourg: Société Philosophique; Louvain: Editions E. Nauwelaerts, 1954), p. 6.

4. For a fuller explanation of these difficult concepts, see W. A. Wallace, "The Intelligibility of Nature: A Neo-Aristotelian View," The Review of Metaphysics, 38 (1984), pp. 33-56, and "Nature as Animating: The Soul in the Human Sciences," The Thomist, 49 (1985), pp. 612-648.

5. For details, see Taylor, "Human Generation," pp. 273-326, for the passages in the Summa in which this process is described. Apparently St. Thomas explained the process somewhat differently in each of the four places in which he discusses it, depending on the biological authority he was using at the time. Probably the fullest treatment is that in De potentia, analyzed by Taylor on pp. 223-266, where Thomas makes more use of medical terminology deriving from Avicenna than in the other places. One text that is particularly helpful is De potentia, q.3, a.12, which reads as follows: "Secundum quod Philosophus probat in XV De animalibus, semen non deciditur ab eo quod fuit actu pars, sed quod fuit superfluum ultimae digestionis; quod nondum erat ultima assimilatione assimilatum. Nulla autem corporis pars est actu per animam perfecta, nisi sit ultima assimilatione assimilata; unde semen ante decisionem nondum erat perfectum per animam, ita quod anima esset forma eius; erat tamen ibi aliqua virtus, secundum quam iam per actionem animae erat alteratum et deductum ad dispositionem propinquam ultimae assimilationi; unde et postquam decisum est, non est ibi anima, sed aliqua virtus animae" (Marietti ed., p. 77).

6. The essentials are provided by Norman Ford, When did I begin?, pp. 65-182, but see also Gabriel Pastrana, O.P., "Personhood and the Beginning of Human Life," The Thomist, 41 (1977), pp. 247-294; A. P. Smith, O.P., "Transient Natures at the Edges of Human Life: A Thomistic Exploration," The Thomist, 54 (1990), pp. 191-227; and W. A. Wallace, O.P., "Nature and Human Nature as the Norm in Medical Ethics," Catholic Perspectives on Medical Morals, ed. E.D. Pellegrino et al., Dordrecht: Kluwer, 1989, pp. 25-53.

7. See his "The Metaphysics of Brain Death, Persistent Vegetative State, and Dementia," The Thomist, 49 (1985), pp. 24-80; also his "Ethics and Brain Death: A Response," The New Scholasticism, 61 (1987), pp. 321-344.

8. See his I-Man: An Outline of Philosophical Anthropology, translated by Marie Lescoe et al. and abridged by F. J. Lescoe and R. B. Duncan (New Britain, Conn.: Mariel Publications, 1985) , pp. 166-186.

9. Shewmon writes, "It should therefore be equally evident that, in the natural context, a person will die (and his spiritual soul will leave the body) the moment his brain dies, irrespective of whether the rest of the body maintains some vegetative integrity of not."-- "The Metaphysics of Brain Death," p. 47; see also pp. 44-48.

10. Ibid., pp. 52-60.

11. In his I-Man: An Outline of Philosophical Anthropology, pp. 177-178.

12. Ibid., p. 179.

13. For a fuller exposition, see W. A. Wallace, "Aquinas's Legacy on Individuation, Cogitation, and Hominization," Thomas Aquinas and His Legacy, ed. D. M. Gallagher (Washington, D.C.: The Catholic University of America Press, 1994), pp. 173-193, which carries forward his investigations in "Nature, Human Nature, and the Norm in Medical Ethics." See also his The Modeling of Nature: A Philosophy of Science Based on a Philosophy of Nature, forthcoming from the Catholic University of America Press.