Donceel relies heavily for his interpretation of Aquinas on the following definition of the soul, which is taken from Aristotle: ‘The soul is the act of a physical organic body which has life potentially.’(S. Th. I, 76, 4, ad I cf. Aristotle, De Anima II, 1) Since ‘‘organic’ in this context means ‘ having organs,’ Donceel interprets this statement as saying that all the organs necessary for the proper operations of the human soul must be in place for a human rational soul to be infused in the body. There are other texts throughout Aquinas’s works to support this understanding.

In the De Anima, for instance, Aquinas says that form gives an act of existing and species to matter according as matter is disposed for the operations of the form, and because the body, which is capable of being perfected by the soul, requires diversity in its parts in order that it may be disposed for the different operations of the soul.(De Anima X, ad 2.)

Similarly, in the Summa Contra Gentiles, Aquinas says the following:

‘[S]ince the intellective soul is the most perfect of souls and its power the highest, its proper perfectible [subject] is a body having great diversity in its organs through which the multiplicity of its operations can be carried out . . .’(S.C.G. II, 86, 4.).

Clearly, however, the embryo is alive, even from conception. But if the embryo is not informed by a human rational soul, what sort of form does it have? Today's thinkers in favor of mediate animation turn again to Aquinas's texts, where the answer is available. Agreeing with Aristotle that the embryo is an animal before it is a man (Aristotle, De Gen. Anim. II, 3) Aquinas says:

‘The embryo has first of all a soul which is merely sensitive, and when this is removed, it is supplanted by a more perfect soul, which is both sensitive and intellectual.’ (S. Th. I, 76, 3, ad 3; cf. S.C.G. II, 89, 9; De Spir. Crea. III, ad 13.)

In order to stay with his understanding of the higher soul as containing within it the operations of the lower forms (S. Th. I, 76, 3, corp.; 4, corp.) he explains further:

And therefore it is said that since the generation of one thing is the corruption of another, it is necessary to say that, both in man and in other animals, when the more perfect form comes, the prior is corrupted, in such a way that the subsequent form has whatever the first hand, and even more. And so through many generations and corruptions it arrives at the ultimate substantial form, both in man and in other animals. And this is apparent to the senses in animals generated from putrefaction. Thus it is said that the intellectual soul, which is simultaneously both sensitive and nutritive, is created by God at the end of human generation, the preexisting forms being corrupted.(S. Th. I,118, 2, ad 2.)

Clearly, then, Aquinas believes that there is a succession of souls, nutritive and sensitive, before the body is finally in a position to be informed by its ultimate form, the human rational soul. How can this change take place?

Given that what comes into being is brought into being by something already existing (reduction from potentiality to act by what is already in act), we need to find the part or being that does the moving or changing in this situation. There is a suggestion in the Summa Contra Gentiles that it is the soul which is responsible for the change.

That which is configured to something is constituted from the action of that to which it is configured: the wax which is configured by the seal receives this configuration from the impression of the seal. Now one sees the bodies of men and other animals to be configured to the proper soul: for the disposition of the organs is such that it suits the operations of the soul exercised through them. The body therefore is formed by the action of the soul; and that is why Aristotle says (De Anima II) that the soul is the efficient cause of the body.(S.C.G. II, 88, 11.)

Aquinas concurs in this (S.C.G. II, 89, 21.) insofar as it makes sense to say that a soul is responsible for the formation of the human body.

Could it be that each successive soul, in some way, manages to transcend itself--that is, to produce by its own power a body capable of receiving a higher form? In this scenario, the original vegetative soul produces a body with the organs necessary for the sensitive soul, which in turn produces a body capable of receiving an intellectual soul. No; such a production is impossible and is not what is indicated by the argument of wax and seal. There is a lack of due proportion of producer to what is produced; a cause cannot be the cause of what is greater than itself. Thus a lower soul, which itself is incapable of higher operations, could not be responsible for the production of organs of higher operations. Nor could it be the case that a lower soul upon further perfection becomes the higher soul, for this would mean: a) that a substantial form is susceptible of degrees, and b) that a rational soul is corruptible, since it would thus be founded in a vegetative and sentient substance. (Spir. Crea. III, ad 13; S.C.G. II, 89, 6 and 7)

It would seem, then, that a rational soul must be responsible. Granting for the sake of argument that a rational soul is the informing principle of the embryo, Aquinas goes through its powers one by one. The power to organize the body cannot come from the embryo's generative powers, for these are operative only in adults and for the purpose of generating others. It cannot be the embryo soul's nutritive power at work here, for in this process nouishment is not assimilated but nutritive material in the mother is brought together to form organs. The power of growth is not responsible, Aquinas claims, because its function is to produce change in quantity, not form. Obviously, the sensitive and intellective powers are inappropriate to such a formation. (S.C.G. II, 89, 8.)

What is Aquinas left with?

It remains therefore that the formation of the body, especially concerning its primary and principal parts, is not from the soul of the thing generated, nor from a formative power acting by virtue of the generated thing, but from [a formative power] acting by virtue of the generative soul of the father, the work of which is to make something like the generating thing according to its species.(S.C.G. II, 89, 8; cf. 89, 21.)

It is thus apparent that, for Aquinas, it is the soul of a parent that is responsible for the development of the embryo body until it is capable of supporting a rational soul.

In Aristotelian physics, whenever we speak of change or movement, there is that which acts and that which is acted upon, i.e., something passive. The same holds true for Aristotelian biology, which Aquinas accepted completely as authoritative. In the abstract, it would serve perfectly well if the active part were donated by the mother, and the passive part by the father. This is not the case, however, according to this early theory of generation. Each parent donates a particular element: the female provides the menstrual blood, the father the semen. From a practical perspective, it makes some sense that the fetal matter be provided by the female, simply in terms of mass. This is precisely what Aquinas says is the case: the female provides the passive element.(S. Th. 1,118,1, ad 4.) If this is so, however, then the father provides the active element. This means it is the father's soul which is responsible for the development of a human body in the embryo. Yet this seems to put us in the awkward position of saying either a) the soul is transmitted in the semen, or b) the action of the father's soul must take place at a distance.

Aquinas devotes a chapter in Summa Contra Gentiles to laying out arguments favoring the transmission of the soul in, or the forming of the soul from, the semen; he spends the next chapter showing how this is impossible. (S.C.G. II, 88 and 89. 35 De An. II, ad 2.)

We have already seen how the soul cannot develop through its stages on its own account. It is equally impossible that the soul be transmitted in the semen, for thus we would have the form before the generation. (S.C.G. II, 89, 4 and 5. 36 S. Th. I,118,1, ad 3; cf. S.C.G. II. 89, 8.)

It is possible, however, for the father's soul to act at a distance, not directly, but through a medium. The generative power of the soul is at work in the instrument of the semen (S. Th. 1,118, 1, ad 4. 37 S.C.G. 11,89,8.) rather as the power of the hunter would be in the arrow which strikes an animal at a distance.

(35) ‘ This active force, which is in the semen, and which is derived from the soul of the generator, is, as it were, a certain movement of the generator's soul itself. . . .’ (36) The semen contains a ‘formative power ‘ which is based ‘ on the vital spirit which the semen contains as a kind of froth.’ (37)  The semen contains an active ‘spirit,’ rather like an active gas or heat or electrical charge, which explains the frothy whiteness of the semen. When this comes in contact with the matter being carried in the female, the menstrual blood, the semen goes to work transmuting this blood, organizing it into a body. For the animal, this transmutation by the semen takes place until the body is disposed to activation by the sensitive soul.

Afterwards, however, through the power of the active principle which was in the semen, the sensitive soul was produced in the thing generated in respect to some principal part, then already the very sensitive soul of the offspring begins to work to the completion of its own body through nutrition and augmentation. (S. Th. I, 188, 1, ad 4)

Only then does the semen, as the instrumental agent, dissipate. This is an important point. Since the species of the subject formed changes, passing from semen to pure blood and on further until it receives the ultimate form of a rational soul, it is necessary that the formative power remain the same throughout the process, from the beginning of the body's formation until the end (when it can support a rational soul). Thus one power is at work through many generations and corruptions. (S.C.G. II, 89, 9.)