The Thomist 67 (2003): 249-78

APPLYING ARISTOTLE IN CONTEMPORARY EMBRYOLOGY

Kevin L. Flannery, S.J.

Pontifical Gregorian University

Rome, Italy

In his book When did I begin?, Norman M. Ford argues that, because up until about the fourteenth day a single human em-bryo can split in such a way that twins (or other sibling groups) result, the embryo during this period cannot be considered a human individual.⁽¹⁾ In historical support of this thesis, he cites Aristotle, who, according to Ford, holds that the sensitive soul--which is a prerequisite for the presence of the rational or properly human soul--enters the embryo some forty days after conception for males, ninety days for females.⁽²⁾ Ford has been challenged on his interpretation of Aristotle by the prominent Aristotelian Enrico Berti, who argues that, if we bear in mind especially Aristotle's application of the doctrine of first act to the beginnings of human life, we must acknowledge that for him the human soul is present from conception.⁽³⁾

In the first part of this essay, I set out Berti's position. In the second part, I expound what I believe to be the correct inter-pretation of Aristotle's embryology, which in certain aspects favors Ford's position. In the third part, I consider what positive use Ford might be able to make of Aristotle's embryology, con-cluding that there is not much: once we make certain adjustments so as to take into account contemporary embryology, Berti's general approach (if not its details) is more applicable. In the fourth part, I acknowledge, however, that any such Aristotelian approach must still deal with Ford's argument regarding twinning. Here some other ideas of Aristotle's prove useful, for Aristotle was aware of the possibility that substances might split, and he constructed his theory of substance accordingly. In the fifth part, I conclude with a few words about a philosophico-theological issue raised by the present approach.

I

Berti's position depends upon three passages: a famous one in De generatione animalium (GA) 2.3⁽⁴⁾ that appears to speak of a succession of souls; a passage in De anima (De an.) 2.3 that compares souls and geometrical figures; and, finally, a passage in Metaphysics (Metaph.) 9.7 that applies the idea of first act to the beginnings of human life.

In GA 2.3, Aristotle says that it is necessary to enquire especially about the sensitive soul (which is the mark of an animal), "does this exist originally in the semen and in the embryo or not, and if it does whence does it come?" He continues:

For nobody would put down the embryo [toV kuvhma (736a32)] as soulless or in every sense bereft of life (since both the semen and the embryo [tav te spevrmata kaiV taV kuhvmata (736a33-34)] of animals live no less than those of plants), and it is productive up to a certain point. That then they possess the nutritive soul is plain (and plain is it from the discussions elsewhere about soul [De an. 2.4 (415a23-26)] 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 . . .⁽⁵⁾ 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 [taV spevrmata kaiV taV kuhvmata (736b8-9)], while not yet separate, must be assumed to have the nutritive soul potentially, but not actually, until (like those embryos [tw'n kuhmavtwn (736b11)] that are separated from the mother) it absorbs nourishment and performs the function of the nutritive soul. For at first all such things seem to live the life of a plant.⁽⁶⁾

And it 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 that it is impossible for them all to preexist 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 connection with the activity of reason.⁽⁷⁾

Berti understands this passage as asserting that the embryo "already possesses a soul, and therefore it cannot have received it--according to Aristotelian doctrine--otherwise than from the sperm." The soul that it "already possesses" is a nutritive soul: "only later is there generated the perceptive--that is, the sensitive--soul, while the question concerning the intellect is left in suspense."⁽⁸⁾ Berti also interprets the remark, "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" (GA 2.3 [736b8-12]), as saying that, "before existing as an individual on its own, i.e., when it is still only seed, the embryo has a soul only in potency; when, however, it exists properly as an embryo, i.e., after the union of the seed furnished by the father with the material furnished by the mother, it has at that point a soul in act."⁽⁹⁾ The soul present after the union of the male and female contributions is a nutritive soul; the sensitive and the rational souls are yet in potenza, awaiting the development of the proper organs (as Aristotle says, "those principles whose activity is bodily cannot exist without a body, e.g., walking cannot exist without feet"). Since the rational soul is present in potency, Berti suggests, it appears that Aristotle is distinguishing it from the intellect, which, even according to this passage, comes in "from the outside."⁽¹⁰⁾

Satisfied that Aristotle is saying in GA 2.3 that right from the beginning the embryo is possessed of a nutritive soul in act and the other two souls in potency, Berti introduces the passage from De an. 2.3. His intention in doing so is to show that the type of potency attaching to the rational soul is such that we can say that the rational soul is present in the embryo right from the beginning.

The cases of figure and soul are exactly parallel; for the particulars subsumed under the common name in both cases--figures and living beings--constitute a series, each successive term of which potentially contains its predecessor, e.g., the square the triangle, the sensory power the self-nutritive. Hence we must ask in the case of each order of living things, What is its soul, i.e., What is the soul of plant, man, beast? . . . For the power of perception is never found apart from the power of self-nutrition . . . . Again, among living things that possess sense some have the power of locomotion, some not. Lastly, certain living beings--a small minority--possess calculation and thought, for (among mortal beings) those which possess calculation have all the other powers above mentioned, while the converse does not hold. . . . Reflective thought [qewrhtikoV" nou'"] presents a different problem.⁽¹¹⁾

It is clear, notes Berti, that Aristotle holds that in any living being there is just one soul.⁽¹²⁾ In human beings, that soul is rational; thus, when Aristotle says in GA 2.3 that the rational (and the sensitive) souls are in the embryo in potency, even at that point we must assume that the real soul is the rational, and that it contains the other two (or, at least, the corresponding faculties). To quote Berti, "Here, as one sees, Aristotle affirms that the superior soul contains in itself the inferior, and not vice-versa. Thus, in the human embryo, one must suppose that there is already contained the soul superior to all, i.e., the rational, which however possesses in act only the nutritive faculty, and in potency the sensitive and rational."⁽¹³⁾ Berti also acknowledges that, for the rational faculty to be able to operate, there is required intelletto teoretico (i.e., qewrhtikoV" nou'"), an issue that Aristotle remands to another forum.

How is this possible? How can a soul that is only in potency in the embryo contain the soul (or the faculty) that is already there in act? This is possible by means of the Aristotelian conception that the soul is "first act of a body which has life in potency."⁽¹⁴⁾ When a person knows a language but is asleep, the language is both in act (first act) and in potency. Before he learned the language, it was (in another sense) in potency; now that it is learned, it is in act. But it can, so to speak, be put in act in a more perfect sense: the person can wake up and use the language. This is sometimes called second act. Thus, right from the beginning, the embryo can be said to have a rational soul in act (i.e., in first act), although this will only be put in act fully (i.e., in second act) once the necessary organs develop, etc.

Berti's third passage, from Metaph. 9.7, is offered in confirmation of the former argument. In it Aristotle is explaining what it means to be in potency for something. That which is potentially a house is not such until all the material is gathered and there is present someone who wants to build a house. Similarly, a doctor cannot cure just any material--he cannot, for instance, cure a dead body; he needs a body that is disposed to be healed. Aristotle discusses here also semen (or seed, spevrma). "But we must distinguish when a thing exists potentially and when it does not; for it is not at any and every time. E.g., is earth potentially a man? No--but rather when it has already become seed, and perhaps not even then, as not everything can be healed by the medical art or by chance, but there is a certain kind of thing which is capable of it, and only this is potentially healthy."⁽¹⁵⁾ This is not the passage quoted by Berti in support of his thesis that the embryo contains the rational soul in (first) act; that passage is the following, which comes a few lines later:

E.g., the seed is not yet potentially a man; for it must be deposited in something other than itself and undergo a change. But when through its own motive principle it has already got such and such attributes, in this state it is already

potentially a man; while in the former state it needs another motive principle, just as earth is not yet potentially a statue (for it must first change in order to become brass).⁽¹⁶⁾

This passage demonstrates, according to Berti, the difference between the semen and the embryo. The former is not yet a man, the latter is: a man in potency, by which is to be understood a man in first act. The difference between the former and the latter is that the latter has its own "motive principle" (ajrchv). Like the material for the house in the presence of the builder and the properly disposed body in the presence of the doctor, from the man in potency a full-grown man will result if no impediment presents itself. Berti concludes, "if the embryo is already in potency, it must already possess in act, as first act, the soul that is proper to the human species, even if it is not capable of exercising immediately all the faculties--that is, it possesses in act only the nutritive faculty and in potency the others."⁽¹⁷⁾

II

In order to deal with Berti's general understanding of Aristotle's embryology, we need now to go back to the first passage, that is, the passage from GA 2.3. We have seen that Berti interprets the remark that, before they are separate, "the semen and the embryo [taV spevrmata kaiV taV kuhvmata (736b8-9)]" have the nutritive soul potentially as saying that the relevant change occurs "after the union of the seed furnished by the father with the material furnished by the mother." Berti rather slides over the fact that in GA 2.3 Aristotle speaks of two types of embryo--or, perhaps better, two stages of the same embryo. The first stage he associates closely with the semen, not only in the piece just

referred to (where he speaks of both the semen and the embryo as "not yet separate") but also at the beginning of the passage: "since both the semen and the embryo [tav te spevrmata kaiV taV kuhvmata (736a33-34)] of animals live no less than those of plants." It is only at the second stage, with respect to which Aristotle mentions the embryo and not the semen, that we have that which "absorbs nourishment and performs the function of the nutritive soul." This is the embryo that has become "separate" from the mother, in the sense that it has bodily integrity and movement of its own.⁽¹⁸⁾ This is consistent with what he says elsewhere in GA, where the embryo requires some time in the presence of the semen--or, more precisely, in the presence of that which it brings: its pneuma--before it launches out on its own. During this time the pneuma is working upon the embryo, but in an external way--that is, in such a way that the embryo cannot be said to have its own "motive principle" or ajrchv and therefore cannot be a man in first act.

In GA 1.21 and 22, Aristotle insists that in the life of the early embryo the action of the pneuma is external. At the beginning of GA 1.21, he asks a number of crucial questions about the relationship between the semen and the menses (the female contribution to animal generation).⁽¹⁹⁾ He wants to know "how it is that the male contributes to generation and how it is that the semen from the male is the cause of the offspring"; and he asks: "Does it exist in the body of the embryo as a part of it from the first [povteron wJ" ejnupavrcon kaiV movrion o]n eujquV"], mingling with the material which comes from the female? Or does the semen communicate nothing to the material body of the embryo but only to the power and movement in it?" (GA 1.21 [729b2-6]; emphasis added). His answer is quite emphatic: "the latter alternative appears to be the right one both a priori and in view of the facts. For, if we consider the question on general grounds, we find that, whenever one thing is made from two of which one is active and

the other passive, the active agent does not exist in that which is made" (GA 1.21 [729b8-11]; emphasis added). He acknowledges, of course, that the formation of the embryo does take place within the female, not the male (GA 1.22 [730a32-b1]); but even this, he says, can be accounted for in terms of his standard examples of crafts: "the carpenter must keep in close connection with his timber and the potter with his clay, and generally all workmanship and the ultimate movement imparted to matter must be connected with the material concerned, as, for instance, architecture is in the buildings it makes" (GA 1.22 [730b6-8]). A craftsman nonetheless always remains external to his artifact (see Metaph. 9.7 [1049a11-12]).

In GA 2.1 Aristotle comes back to this issue, suggesting at first that the operation of the semen must be internal to the embryo: "Now it would appear irrational to suppose that any of either the internal organs [splavgcnwn (734a2)] or the other parts is made by something external, since one thing cannot set up a motion in another without touching it, nor can a thing be affected in any way by anything that does not set up a motion in it" (GA 2.1 [734a2-4]). (The parts of the body mentioned here would include the so-called homoeomerous parts such as blood, flesh, and the material of bones.)⁽²⁰⁾ After mentioning some possible objections, he says that it is necessary to make some distinctions; in particular, we must determine in what sense it is impossible that the parts might be generated by something external (GA 2.1 [734b4-7]): "For if in a certain sense they cannot, yet in another sense they can."

It is possible, then, that A should move B, and B move C; that, in fact, the case should be the same as with the automatic toys. For the parts of such toys while at rest have a sort of potentiality of motion in them, and when any external force puts the first of them in motion, immediately the next is moved in actuality. As,

then, in these automatic toys the external force moves the parts in a certain sense (not by touching any part at the moment, but by having touched one previously), in like manner also that from which the semen comes, or in other words that which made the semen, sets up the movement in the embryo and makes the parts of it by having first touched something though not continuing to touch it. In a way it is the innate motion that does this, as the act of building builds the house. (GA 2.1 [734b9-17])

When Aristotle says here that "that from which the semen comes, or in other words that which made the semen, sets up the movement in the embryo," he has in mind his doctrine that the semen (and its pneuma) function as tools of the father.⁽²¹⁾ The "automatic toys" are a bit of a mystery, but it is clear that, having once received an impulse, they could prolong that motion and thus appear to move themselves.⁽²²⁾ The upshot of the passage is clearly that, at least during the initial stages of embryonic development, what might appear to be action on the part of the embryo is not its own; that is, the embryo has not yet the "motive principle" mentioned in Metaph. 9.7, Berti's third passage.

This is not to say, however, that the movement (or development) of the early embryo is to be wholly and simply assimilated to the mechanical movement of automatic toys. In GA 2.5, just after referring once again to the automatic toys, Aristotle says, "When some of the natural philosophers say that like is brought to like, this must be understood, not in the sense that the parts are moved as changing place, but that they stay where they are and the movement is a change of quality [ajlloiouvmena,

(741b12)] (such as softness, hardness, color, and the other differences of the homoeomerous parts)" (GA 2.5 [741b9-14]). It is not at all clear why Aristotle is conceding anything here to "the natural philosophers" (he has just earlier rejected the utility of the principle "like is brought to like" (GA 2.4 [740b12-14]); but it is clear that he does not understand the action of the semen as sheerly mechanical. It is external but it does bring about a real alteration (ajlloivwsi") in the material provided by the mother, not just a shifting of parts. The alteration of which he speaks is again, and significantly, alteration of the characteristics of the homoeomerous parts. These are, of course, to be distinguished from the nonhomoeomerous parts such as the heart, brain, eyes, etc., which are, according to GA 2.3, required for the presence of certain faculties of the soul: "Plainly those principles whose activity is bodily cannot exist without a body, e.g. walking cannot exist without feet" (736b22-24). Nothing that Aristotle says excludes, however, the possibility that the semen's work upon the maternal material might result in the formation of organs (splavgcnwn; see GA 2.1 [734a2]).

But the most important piece of evidence for the present interpretation comes in GA 2.4. Having explained that the action of the semen upon the female material is like the way that rennet acts upon milk in order to curdle (or "set": sunivsthsi [739b24]) it, Aristotle again makes the point that, at the beginning, the embryo has the life of a plant (GA 2.4 [739b34]). The plant, which contains a "motive principle" (ajrchv), only eventually produces the analogues of organs: the shoot, the root, etc. In an animal, it is with one of these organs that come along eventually that one is to associate the motive principle of the organism itself: the heart.

So also in the embryo all the parts exist potentially in a way, but the first principle is furthest on the road to realization. Therefore the heart is first differentiated in actuality. This is clear not only to the senses (for it is so) but also on theoretical grounds. For whenever the young animal has been separated from both parents it must be able to manage itself, like a son who has set up house away from his father. Hence it must have a first motive principle [dei'

ajrchVn e[cein (740a7-8)] from which comes the ordering of the body at a later stage also, for if it is to come in from outside at a later period to dwell in it, not only may the question be asked at what time it is to do so, but also we may object that, when each of the parts is separating from the rest, it is necessary that this principle should exist first from which comes growth and movement to the other parts. (GA 2.4 [740a1-13])

Notable in this passage are the words "whenever the young animal has been separated"; they correspond to the remark in GA 2.3 about the semen and the embryo which, "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" (GA 2.3 [736b8-12]). Aristotle says in the passage we are now examining that only once this separation, which coincides with the formation of the heart, has occurred do we have the all-important motive principle. A couple of chapters later, Aristotle says that it is the heart that organizes the whole subsequent life of the organism: "if there is anything of this sort which must exist in animals, containing the principle and end of all their nature [toV pavsh" e[con th'" fuvsew" ajrchVn kaiV tevlo" (742b1)], this must be the first to come into being--first, that is, considered as the moving power, but simultaneous with the whole embryo if considered as a part of the end" (GA 2.6 [742a37-b3]).

Putting all these ideas together, the scheme that forces itself upon us is one in which the animal embryo has initially the life of a plant, although with this difference: a plant has an internal motive principle, whereas an embryo is moved along by the continued action of the semen, which is only internal in the way that the motion of automatic toys is internal. Eventually, however, the principle of the animal's own nutritive life is constructed out of the material provided by the mother. This principle is the heart. Aristotle very clearly associates it with the animal's nutritive life in lines coming shortly after the longer passage just quoted:

Therefore it is that the heart appears first distinctly marked off in all the sanguinea, for this is the first principle [ajrchv] of both homoeomerous and nonhomoeomerous parts, since from the moment that the animal or organism

needs nourishment, from that moment does this deserve to be called its principle [ajrchvn (740a19)]. For that which exists grows, and the nutriment, in its final stage, of an animal is the blood or its analogue, and of this the blood-vessels are the receptacle, and that is why the heart is the principle of these also. (GA 2.4 [740a17-23])

We are not therefore to associate the nutritive life of the animal with the nutritive life one sees in the early developing embryo. The former is related more to plant life, and the nutritive faculty in animals is quite different. As Aristotle says in De an. 2.3, "It is . . . evident that a single definition can be given of soul only in the same sense as one can be given of figure. . . . Hence we must ask in the case of each order of living things, What is its soul, i.e. What is the soul of plant, animal, man?" (De an. 2.3 [414b20-33]). It is true that the nutritive faculty of a plant can be said to fall under the same definition as the nutritive faculty of man, but this would only be a logical definition: the nutritive life of a plant is radically different from that of a man (similar to the way that a triangle is radically different from a square).⁽²³⁾ In fact, it is difficult even to say that the early embryo has a nutritive soul, although it does, as we have seen, "seem to live the life of a plant" (GA 2.3 [736b12-13]). Immediate after the passage about the automatic toys in GA 2.1, Aristotle says of the pneuma, "Plainly, then, while there is something [i.e., the pneuma] which makes the parts, this does not exist as a definite object [tovde ti], nor does it exist in the semen at the first as a complete part."⁽²⁴⁾ If the early embryo had a soul, it would inform its body and together they would constitute a tovde ti in such a way that also the form could be said to be a tovde ti (see Metaph. 7.3 [1029a27-30]). As things are, according to Aristotle, the pneuma never really enters into the material presented by the mother. A genuine tovde ti--and a

genuine soul--arrives on the scene only with the appearance of the heart.

When does all this happen? In Historia animalium, Aristotle says that "[i]n the case of male children the first movement usually occurs on the right-hand side of the womb and about the fortieth day, but if the child be a female then on the left-hand side and about the ninetieth day" (HA 7.3 [583b3-5]). "About this period," he says, "the embryo begins to resolve [scivzetai] into distinct parts, it having hitherto consisted of a fleshlike substance without distinction of parts [a[narqron sunevsthke krew'de"]" (HA 7.3 [583b9-11]). But Aristotle warns us not to look for precise answers to such questions, that is, answers that apply across the board (HA 7.3 [583b5-9]). Although here in Historia animalium he says that, in these first days of the quickened embryo, "all the limbs are plain to see, including the penis, and the eyes also, which as in other animals are of great size" (HA 7.3 [583b18-20]), we have also seen him say that, although "all the parts exist potentially in a way at the same time . . . the first principle is furthest on the road to realization. Therefore the heart is first differentiated in actuality" (GA 2.4 [740a2-4]; see also PA 3.4 [666a20-23]). Two chapters later, he says that the heart is followed immediately by the head: "that part which contains the first principle comes into being first, next to this the upper half of the body. This is why the parts about the head, and particularly the eyes, appear largest in the embryo at an early stage, while the parts below the umbilicus, as the legs, are small; for the lower parts are for the sake of the upper, and are neither parts of the end nor able to form it" (GA 2.6 [742b12-17]).

So, the organs necessary for the various functions of the animal appear more-or-less at the same time: the heart first, but the head and the eyes follow shortly thereupon. We have already seen that the heart is associated with nutrition, but it is also tied up with sensation: "the motions of pain and pleasure, and generally of all sensation, plainly have their source in the heart, and find in it their ultimate termination" (PA 3.4 [666a11-13]; see also GA 2.6 [742a32-33], regarding locomotion). Indeed, as we have already

seen, the heart is in animals the "principle and end of all their nature" (GA 2.6 [742b1]; emphasis added). And the organ corresponding to the sense of touch is the body itself (GA 2.6 [743b37-744a1]), which would seem to have its human beginning simultaneously with the heart.

This is all evidence in support of Berti's thesis that the various faculties arrive at one time. Even if the primary sense organs appear after the heart, the heart is involved not just in nutrition but also in sensation. As for the rational part, since it requires no physical organ (although in men it presupposes the sensitive functions), there is no difficulty locating its inception also at this beginning, that is, when the heart is first formed. This would be consistent with Aristotle's remark in De an. 2.3, comparing the functions of the soul to geometrical figures: "for the particulars subsumed under the common name in both cases--figures and living beings--constitute a series, each successive term of which potentially contains its predecessor" (414b29-30).⁽²⁵⁾

The present interpretation also partially supports Berti's use of Metaph. 9.7, regarding the embryo as first act. This becomes more apparent, however, if we make a slight revision to the text that Berti gives us. In his translation, the seed is said not to become

potentially a man (i.e., a man in first act) until it is first "deposited in something other than itself and [undergoes] a change." The word "deposited" in the phrase "deposited in something other than itself"--which suggest that the man is there in first act (in Berti's words) "after the union of the seed furnished by the father with the material furnished by the mother"⁽²⁶⁾--corresponds to the Greek word pesei'n inserted by Ross at 1049a15. It appears in no manuscript, although pseudo-Alexander includes it in a paraphrase of the passage.⁽²⁷⁾ Read without the word pesei'n, the passage runs as follows:

[T]he seed is not yet potentially a man; for it must further undergo a change in a foreign medium. But when through its own motive principle it has already got such and such attributes, in this state it is already potentially a man; while in the former state it needs another motive principle. (Metaph. 9.7 [1049a14-17])

Ross argues that here Aristotle "is not taking account of his own view that the spevrma forms no part of the matter of the offspring but is its formal and efficient cause";⁽²⁸⁾ but another explanation might be that Aristotle is using the word "seed" (spevrma, Metaph. 9.7 [1049a14]) in the generic sense that does not exclude the female contribution or the embryo itself.⁽²⁹⁾ In any case, the passage without the word pesei'n no longer suggests that the crucial moment is the moment when the seed falls onto the menses but says only that it (or, more correctly, the embryo), must "undergo a change"--sometime or another. This moment is to be associated with the first presence of the motive principle. And this latter is surely to be associated with the heart, which becomes visible

sometime around the fortieth day in males, the ninetieth in females (HA 7.3; GA 2.4, 6). Berti is correct to apply the Aristotelian idea of first act and Metaph. 9.7 in particular to the question of the onset of the human soul; he errs, in my opinion, regarding the timing of this onset.

III

Does all this play into Norman Ford's hand? Can Ford call Aristotle in support of his thesis that the human individual is not present at conception? In a way yes, in a way no. In order to understand the impact of Aristotle's embryology (both biological and metaphysical) on Ford's theory, we need to understand why Aristotle puts the onset of the motive principle at forty days (or so) rather than earlier.

Let us go back to Aristotle's "crucial questions" asked at the beginning of GA 1.21 about the relationship between semen and menses. As we have seen, he says there that we must ask

how it is that the male contributes to generation and how it is that the semen from the male is the cause of the offspring. Does it exist in the body of the embryo as a part of it from the first, mingling with the material which comes from the female? Or does the semen communicate nothing to the material body of the embryo but only to the power and movement in it? (GA 1.21 [729b1-6])

It may not be immediately apparent, but in this passage Aristotle is peering down the alley where eventually genetic theory was to be discovered, and turning away for lack of light. He is asking whether there is something physical in the male semen that combines with the female element to form the embryo. He has in mind theories of his day, collectively known as "pangenesis," according to which the semen contained something drawn from every part of the body that would, after copulation, grow into a full-sized animal.⁽³⁰⁾ Its advocates sometimes spoke of a small

animal (zw/'on mikrovn [722b4-5]) (or, in humans, the so-called homunculus) passed along in the semen.⁽³¹⁾ Aristotle had no evidence that there was any such object in the semen, nor any such resulting articulation in the embryo itself, up until at least the fortieth day. Until that point, he says, the embryo is "fleshlike" (krew'de", HA 7.3 [583b10-11]), which puts it among the homoeomerous parts (Mete. 4.10 [388a16]; PA 1.1 [640b18-20]). It is "without distinction of parts" (a[narqron, HA 7.3 [583b9]). Aristotle does say that within the female element are found all the parts of the animal in potency (GA 2.4 [740b18-21]), even those that distinguish the sexes (GA 2.3 [737a24-25]); but this must mean that they are found in potency the way that a bed is found in potency in the wood that stands before the carpenter, for Aristotle denies elsewhere that sex is determined at conception (GA 1.18 [723a23-b3]; 4.1 [763b26-27]). For him, sex differentiation depends upon the strength of heat in the semen (GA 4.1 [766a16-22]), which goes to work on the menses, only eventually producing organs, as we have seen.

Most interestingly, there were theories available in the intellectual culture within which Aristotle worked that, at least in

certain respects, come closer than his own to modern genetic theory. Aristotle himself is our best source regarding these theories. He recounts, as we have seen, that "some say that [the semen] comes from the whole of the body" (GA 1.17 [721b11-12]). (Since some such thinkers were open to the idea that semen was produced by both parents [GA 1.17 (721b6-8)], "semen" here need not mean male semen.) One of their reasons for believing this was that children resemble their parents: "for the young are born like them part for part as well as in the whole body; if then the coming of the semen from the whole body is cause of the resemblance of the whole, so the parts would be like because it comes from each of the parts" (GA 1.17 [721b20-24]). Since the semen comes from all parts of the body, it would contain somehow all the traits that are passed on to the children. Empedocles even had a theory that exploited the idea of a part corresponding to a companion part, the two contributing to the whole progeny; he held, that is, that "there is a sort of tally in the male and female, and that the whole offspring does not come from either, 'but sundered is the fashion of limbs, some in the man's, <some in the woman's seed hidden>'" (GA 1.18 [722b10-12])⁽³²⁾ This is not a bad way of representing the way in which the male and female gametes both contribute to the genetic material of the zygote.

There is much that is correct in such theories. As we now know, there does exist right from the beginning in the embryo that which determines individual traits of the eventual progeny: DNA. It is not drawn from the whole body, originating rather in the appropriate organs of the parents' bodies, but it does contain information and "instructions" corresponding to every part of the

parents' bodies. Moreover--and this is important--DNA is physically present in the gametes and then in the embryo from the moment of conception. Thus, when Aristotle asks his "crucial question" about how the male contributes to generation--"Does it exist in the body of the embryo as a part of it from the first [wJ" ejnupavrcon kaiV movrion], mingling with the material which comes from the female?" (GA 1.21 [729b2-4])--and subsequently answers no, the semen "communicates nothing to the material body of the embryo but only to the power and movement in it" (GA 1.21 [729b4-6; see also 729b8-9]), he is wrong, at least regarding the existence of something of the semen in the body of the embryo.

But Aristotle also had good reasons for rejecting the pangenesis of his day. It was often quite crudely physicalist. Anaxagoras, an exponent of the theory, asks, "How can hair come from not-hair?";⁽³³⁾ so he evidently thought that hair somehow found its way into the semen. Moreover, Aristotle, displaying not a little genetic sophistication, argues that the proponents of pangenesis had no way of explaining how a child can resemble a remote ancestor: "for the resemblances recur at an interval of many generations, as in the case of the woman in Elis who had intercourse with a negro; her daughter was not negroid but the son of that daughter was" (GA 1.18 [722a8-11]).⁽³⁴⁾ They have no way of explaining either, says Aristotle, how a full plant can grow from just a cutting (ibid. [722a11-14]).

Aristotle offers many other such arguments, some stronger than others, but his basic one is this: the proponents of pangenesis had no concrete evidence for their theory (see GA 2.1 [734a20-25]). When one takes a look at the early (aborted) embryo, one sees no differentiation (HA 7.3 [583b20-21]). Nor did Aristotle see such articulation in the semen or in the menses (GA 1.20

[729a20-33]; 2.1 [734a33-b3]). Therefore, he sought a theory that would explain how from unarticulated matter a fully articulated fetus and human person could grow. The theory involved, naturally enough, a number of his favorite concepts: potency and act, passion and motion, matter and form. The semen contributes nothing physical to the embryo but only form and movement, in the transmission of which it serves (along with pneuma) as tool of the father. To use a standard Aristotelian example (see, for instance, GA 1.18 [722a30-b1]), think of a word. It can be broken down into syllables and ultimately letters (which we can assume are purely physical). That which the semen passes on is the arrangement (sunqevsew" [722a35]) of the letters, not the letters. In the passage where he asks his "crucial questions," he speaks of the semen's bringing shape or form (morfhVn) to the menses, as well as "power and movement" (duvnami" kaiV kivnhsi", GA 1.21 [729b4-8]). In the succeeding chapter, comparing the action of the semen to the work of a carpenter, he says that it transfers to the material "the shape and the form" (hJ morfhV kaiV toV ei\do", GA 1.22 [730b14]).

We now know that DNA also involves a certain arrangement of its various constituent elements (the nucleotide pairs, adenine-thymine and guanine-cytosine), which play the primary role in relaying genetic information. Neither an arrangement nor information is, strictly speaking, physical. Aristotle had this right. What he did not know was that, in actual fact, the genetic information that enters into the embryo from the father is attached to physical objects: nucleotides and amino acids. According to Aristotle, the father's contribution is just form, combined somehow with propulsion; the female contribution, as we have already noted, is (relatively) undifferentiated matter. The organs of the embryo arrive--but only eventually--because the power and movement passed along by the semen (and pneuma) impose form--human form--upon this matter.

But had Aristotle known what modern scientists know, what would he have said? He would certainly have had no trouble with the idea that genetic information is attached to matter; if

anything, it was an embarrassment for his actual theory that form is transmitted by way of something as vague as pneuma (which is not a tovde ti, as we saw above when looking at GA 2.1[734b17-19]). Moreover, there is certainly no reason he would have felt obliged to jettison his theory of potency and act. It must have been difficult for him explain how pneuma (of which he gives no systematic analysis) can carry power and movement;⁽³⁵⁾ and, no doubt, he felt himself on more stable philosophical ground once he was able to identify an organic body moving along by means of its own "motive principle" (GA 2.4 [740a1-13]). Once upon the secure terrain of physical causation, he could invoke potency and act with impunity. Physical genetic material (DNA) was what he wanted all along but could not find.

Can then Norman Ford call Aristotle in support of the argument that the human individual is not present at conception? As I suggested above: in a way yes, in a way no. Ford is correct to say that Aristotle does not hold that the human soul is present in the embryo from the moment of conception. But, in the end, this is a support only for someone who wants to maintain an antiquated biology that sees both the male and the female contributions to human generation as lacking physical articulation. Ford is no such person. Once we make the adjustments, taking into account advances in contemporary biology and embryology, we can make good use especially of Aristotle's idea of first act, as expounded by Berti. The result is a philosophically rich account of how the human soul can be present in the conceptus from the beginning, that is, from the moment when the two gametes meet.

IV

We are not out of the woods yet. As we have seen, Ford's central argument is that, since up until about the fourteenth day a single human embryo can split, thereby producing twins, the

embryo during this period cannot be considered a human individual. Aristotle can be of use here too since, as it happens, he has quite a bit to say about such splitting.⁽³⁶⁾ It will serve our purposes to look at a couple of the pertinent passages. They concern not human embryos but rather plants and annelids such as earthworms; but, for reasons I shall explain below, we can apply these Aristotelian ideas in considering the twinning that occurs with human embryos.

The first passage is found in De an. 2.2. Aristotle has noted that the soul is the source of such phenomena as the "powers of self-nutrition, sensation, thinking, and movement." He then asks whether such powers (or faculties) are located spatially.

In the case of certain of these powers, the answers to these questions are easy, in the case of others we are puzzled what to say. Just as in the case of plants which when divided are observed to continue to live though separated from one another (thus showing that in their case the soul of each individual plant was actually one, potentially many), so we notice a similar result in other varieties of soul, i.e. in insects which have been cut in two; each of the segments possesses both sensation and local movement; and if sensation, necessarily also imagination and appetition; for, where there is sensation, there is also pleasure and pain, and, where these, necessarily also desire. (413b15-24)

Thus the nutritive and sensitive powers are spatially fixed by the material they happen to inform. More important for present purposes, however, is the way in which this happens. As Aristotle says, "in their case the soul of each individual . . . was actually one, potentially many" (ou[sh" th'" ejn aujtoi'" yuch'" ejnteleceiva/ meVn mia'" ejn eJkavstw/ futw/', dunavmei deV pleiovnwn [413b18-19]).

In the prologue to Plato's Parmenides, the character Parmenides leads his interlocutor Socrates into aporia by pursuing the question whether, when something participates in an Idea, it participates in all of it or just a part (Parm. 131A5-6). Socrates is inclined to say, "in all of it," but then Parmenides proposes the example of a sail (131B8). He suggests that Socrates's subsuming the many men under one Idea in which they participate wholly is like covering them with a sail and then affirming that each man

stands under all of the sail instead of under just a part of it (Parm. 131C2-8). Socrates is thus forced to assert, "the Ideas themselves are divisible and the things that participate in them participate in a part; in any object there will be not be the entire Idea but a part" (Parm. 131C5-8); but this position too is rejected by Parmenides.

Aristotle's remark in De an. 2.2 is, in effect, a response to this aporia. Whereas Plato, on account of such aporiai, is forever inclined to push forms (his "Ideas") off into a separate suprasensible realm, Aristotle insists that in material substances, in any case, although form is distinct from matter, it is nonetheless tied to it in such a way that it follows its appropriate matter in its entirety. It can do this since form is not subject to the same causal processes as matter. It is neither generated nor does it corrupt (Metaph. 7.9 [1034b8]; 8.3 [1043b14-18]; 12.3 [1069b35]); it is indivisible (Metaph. 7.8 [1034a8]); in short, it is not potentially something and then that something in act. It is rather composites of matter and form that are generated and corrupt and are subject to other changes.

One of the advantages of this theory is that, using it, Aristotle can explain what is happening when, for instance, an earthworm is split in two and suddenly becomes two earthworms--two different substances. This is not a problem for him since form is not the sort of thing that can split. In order for splitting to occur, obviously, there would have to be something that can be first one and later two; but a form, qua form, is always one: the form of man recognized in Socrates is the same form recognized in Parmenides. Now, although Socrates and Parmenides are thus one in species, they are two in number, as Aristotle never tires of saying (e.g., Metaph. 5.6 [1016b32-33]; 7.8 [1034a7-8]). Number, however, pertains to their matter, not to their form as such. Thus, Aristotle can say (in the above passage) that, in the case of plants and certain divisible animals, each individual is "actually one, potentially many." After a split occurs--which splits can only occur on the material level--the whole of the form (i.e., the "whole nature" of the form) is found in both pieces, now

spatially separated. This is no more mysterious than when we divide a lump of gold in two and notice that the resulting pieces are entirely gold. When we divide a material substance, we divide material, not form. We effect two composites sharing fully in the same form; we do not split a form.

Our second passage is taken from Metaph. 7.16. In it Aristotle is explaining that the actuality--the act--of a substance excludes there being other acts within it. His examples have mostly to do with parts that, because of the complexity of their governing substance, can never have independent existence; but he also mentions divisible animals.

Evidently even of the things that are thought to be substances, most are only potentialities,--e.g. the parts of animals (for none of them exists separately; and when they are separated, then too they exist, all of them, merely as matter) and earth and fire and air; for none of them is one, but they are like a heap before it is fused by heat [pefqh'/ (1040b9)] and some one thing is made out of the bits.⁽³⁷⁾ One might suppose especially that the parts of living things and the corresponding parts of the soul are both, i.e. exist both actually and potentially, because they have sources of movement in something in their joints; for which reason some animals live when divided. Yet all the parts must exist only potentially, when they are one and continuous by nature--not by force or even by growing together, for such a phenomenon is an abnormality. (1040b5-16)

Obviously, in this passage unity is a key concept. Something can only be in act as a substance if it is a unity of its parts. This unity prevents the parts from being substances in their own right--that is, from being substances in act; they must remain in potency. Most such parts do not in fact become substances in act when the unity of the original substance is lost. When a man dies, his disparate parts become not individual men but "a heap." But in some cases the potentiality in the part does become act. What is it that allows this to happen? There are present in the parts, such as the parts of earthworms, sources of movement (tw/' ajrcaV" e[cein kinhvsew" [1040b12-13]), that can survive a division. While still within the body of the original earthworm, they are in potency;

after a division, the ancillary sources of movement become central sources and we have new substances.

We have now a number of Aristotelian doctrines before us; we must see whether they can be applied in the analysis of the early embryo. There is a prima facie case against such an application: Aristotle himself does not speak of twinning as occurring in this fashion. This is easily dealt with. Knowing little about the true nature and functioning of the early embryo, he had no idea that embryos might split in the early days of gestation. For him, in other animals, twins resulted from large amounts of menses; in man, "because of the moisture or heat of his body" (GA 4.4 [772a18-22, b3-4]). Taking into account what we now know about monozygotic twinning, we can do what Aristotle did not know how to do: apply his account of how some animals can be divided to the early human embryo.

In De juventute et senectute 2, Aristotle remarks: "Divisible animals are like a number of animals grown together, but animals of superior construction behave differently because their constitution is a unity of the highest possible kind" (Juv. 2 [468b9-12]). Among "animals of superior construction" Aristotle no doubt includes man, but we now know that the clause excluding them from division and survival applies only once the embryo gets passed its first fourteen days or so (past the "primitive streak stage").⁽³⁸⁾ Before that period we can indeed say that men are "like a number of men grown together."⁽³⁹⁾ As we

have seen, in saying this of dividing animals, Aristotle does not mean that there are many actual animals in the original; he is referring rather to potential animals. All the elements required to produce a fully grown animal out of a part are present in that part. Once a split occurs, that part is an animal in first act. So also with the early human embryo. Before the split, it is one, but potentially two (or more) since at least certain of its parts have within them that which is required to become unities with motive principles of their own. In response to Ford, therefore, we can say that there are no Aristotelian reasons independent of his primitive biology to say that the earliest human embryo is not a human being. If an earthworm is split, resulting in two earthworms, we do not deny that the original earthworm was an earthworm.

V

This still leaves a large number of difficulties to be dealt with. I will address just one of them, an important one, which is partially theological in character. Does not this approach constitute traducianism, that is, the belief that the soul is passed on from one human individual to another rather than created by God? Traducianism has long been an issue associated with Aristotelian embryology due to statements in Aristotle suggesting that the soul of the embryo comes from the father (see, for instance, GA 2.4 [738b25-26]). For those who wish to combine Aristotle's properly philosophical ideas with modern biology, however, traducianism with respect to progenitor and conceptus is no longer an issue. It is now clear that the male and female gametes are involved equally in conception--that is, in the production of the genetically structured entity to which, on Aristotelian grounds, one can assign a human soul in first act. Even if someone should still want to say that both male and female contribute somehow to the embryo's soul, the latter is, at the moment of conception, undoubtedly a "new thing," coming into existence, to a significant extent by chance, when the oocyte

comes in contact with one of the sperm racing toward it. But in the production of monozygotic twins we have a different situation, which does indeed raise the specter of traducianism. Let us call an original embryo "embryo-1"; let us say too that it is of four cells that split into "embryo-2" and "embryo-3," containing two cells each (the two embryos then developing in the usual way). In the production of these monozygotic twins, the life and the motive principles of embryos-2 and -3 clearly come from embryo-1. Moreover, since it now appears that monozygotic twinning of humans can be provoked, it would also appear that a scientist working in a laboratory (or operating room) could be the immediate cause of such a transfer of life or motive principle.⁽⁴⁰⁾

Here perhaps Thomas Aquinas can be of some help. There are a number of passages in which he allows that the nutritive and sensitive souls might arrive in the embryo "by transmission"--that is, in a way that would constitute an acceptable sort of traducianism. (Thomas holds that these souls enter the embryo as parts of a succession: nutritive, sensitive, then rational.)⁽⁴¹⁾ In the second book of his commentary on the Sentences, for instance, after discussing the positions of some other philosophers (including Plato, Avicenna, and Themistius), he cites Aristotle's De an. and GA and says, "the position of Aristotle is much more reasonable, for nothing begins or comes about or is generated except according to the way in which it has its being; and, therefore, we concede that the sensitive soul and the nutritive are by transmission."⁽⁴²⁾ In a passage in the Summa contra Gentiles, he actually says that such transmission occurs in worms (in

animalibus anulosis) that live after being divided.⁽⁴³⁾ Exploiting these ideas, a philosopher intent on avoiding traducianism could acknowledge that, when embryo-1 splits, it transmits nutritive and sensitive souls to embryo-2 and embryo-3. He could then employ another type of explanation for the rational soul, perhaps arguing that the operations of the rational soul "do not come about by means of any corporeal organ" (ScG II, c. 68) and that, therefore, the rational soul's generation must be of a different, nonphysical order.⁽⁴⁴⁾ Since the creation of the rational soul is independent of the physical development of the embryo, except that the rational cannot be present before the sensitive, such a philosopher could say that, when the split occurs, God creates two new souls. These new souls are, of course, rational souls; and, since a man is possessed of just one soul, the nutritive and the sensitive souls are present within these rational souls as faculties (we recall De an. 2.3 [414b28-415a12], Berti's second passage). But the nutritive and sensitive faculties were indeed transmitted by embryo-1.

As I have said, there remain a number of difficulties to be dealt with, having to do especially with the relationship of the rational soul of embryo-1 to that of its two "progeny" and with the threat of an unacceptable type of dualism, given that we are suggesting that a new soul (for instance, the soul of embryo-3) comes to inhabit a body--or at least part of a body--previously occupied by another soul (that of embryo-1). But I must leave these issues to another occasion. For the present, however, I believe we can say that, with a number of adjustments made necessary by

scientific advances, Aristotle can be applied fruitfully in contemporary embryology.⁽⁴⁵⁾