Summa contra Gentiles2.72 Chapter LXXII. That the whole soul is in the whole body and in each part thereof

From the same premisses we can prove that the whole soul is in the whole body, and in each part thereof.

For the proper act must be in its proper perfectible subject. Now the soul is the act of an organic body, not of one organ only. Therefore it is in the whole body, and not only in one part, according to its essence whereby it is the form of the body.

And the soul is the form of the whole body in such a way as to be also the form of each part. Eor were it the form of the whole and not of the parts, it would not be the substantial form of that body: thus the form of a house, which is the form of the whole and not of each part, is merely an accidental form. That it is the substantial form both of the whole and of the parts, is clear from the fact that both the whole and the parts take their species from it. Wherefore, when it departs, neither whole nor parts retain the same species: for the eye or flesh of a dead person are only so called equivocally. Accordingly if the soul is the act of each part, and an act is in the thing of which it is the act, it follows that it is by its essence in each part of the body.

That this applies to the whole soul is evident. For since whole denotes relation to parts, it follows that whole is taken in various senses, according to the various meanings of parts. Now part is taken in two ways. First, forasmuch as a thing is divided according to quantity; thus two cubits is a part of three cubits. Secondly, forasmuch as a thing is divided by a division of its essence; thus form and matter are said to be parts of a composite. Hence a whole is spoken of in reference both to quantity and to essential perfection. Now whole and part in reference to quantity are not applicable to forms save accidentally, namely in so far as they are divided when the quantitative subject is divided. On the other hand whole or part in reference to essential perfection is found in forms by their very nature. Speaking then of this kind of totality, which is applicable to forms by their very nature, it is clear regarding every form that the whole of it is in the whole (subject), and the whole of it in each part thereof: for just as whiteness is in a whole body in respect of the whole essence of whiteness, so is it in each part thereof. It is otherwise with the totality which is ascribed to forms accidentally: for in this sense we cannot say that the whole whiteness is in each part. Accordingly, if there be a form that is not divided when its subject is divided, as the souls of perfect animals, there will be no need for a distinction, since only one totality is applicable to them: and we must say absolutely that the whole of it is in each part of the body. Nor is this difficult to conceive for one who understands that the soul is not indivisible in the same way as a point, and that an incorporeal is not united to a corporeal being in the same way as bodies are united together, as we have expounded above.

Nor is it inconsistent that the soul, since it is a simple form, should be the act of parts so various. Because the matter of every form is adapted to it according to its requirements. Now the more noble and simple a form is, the greater is its power: and consequently the soul which is the noblest of the lower forms, though simple in substance, is manifold in power and has many operations. Wherefore it needs various organs in order to accomplish its operations, of which organs the various powers of the soul are said to be the acts; for instance sight of the eye, hearing of the ears, and so forth. For this reason perfect animals have the greatest variety of organs, while plants have the least.

This explains why certain philosophers have stated that the soul is in some particular part of the body: thus Aristotle (De Causa Motus Anim.y says that it is in the heart, because one of its powers is ascribed to that part of the body. For the motive power, of which Aristotle was treating in that book, is chiefly in the heart, by which the soul communicates movement and other like operations to the whole body.