Summa contra Gentiles2.56 Chapter LVI. In what way it is possible for an intellectual substance to be united to the body

Now since it has been shown that an intellectual substance is neither a body, nor a force dependent on a body, it remains for us to inquire whether an intellectual substance can be united to the body.

In the first place it is clear that an intellectual substance cannot be united to the body by way of a mixture. For things that are mixed together must needs be altered in relation to one another. And this does not happen except in those things whose matter is the same, and which can be active and passive in relation to one another. But intellectual substances have no matter in common with corporeal substances, since they are immaterial, as we have proved above. Therefore they cannot be mixed with bodies.

Further. Things that are mixed, remain not actually but only virtually, after the mixture is made: for were they to remain actually it would not be a mixture but only an accumulation, wherefore a body formed by a mixture of elements is no one of them.' But this cannot possibly happen to intellectual substances, since they are incorruptible, as we have proved above.

Therefore an intellectual substance cannot be united to the body by way of a mixture.

It is likewise evident that an intellectual substance cannot be united to the body by way of contact properly so called. For contact is only between bodies, since things are in contact when they come together at their extremes, as the points, lines, or superficies which are the extremes of bodies. Therefore it is not possible for an intellectual substance to be united to the body by way of contact.

Hence it follows that neither by continuity, nor fellowship or connecting tie is it possible for one thing to result from an intellectual substance with the body. For none of these is possible without contact.

And yet there is a kind of contact whereby it is possible for an intellectual substance to be united to a body. For natural bodies are mutually alterative when in contact with one another: so that they are united to one another not only as to their quantitative extremes, but also in like manner to qualities or forms, when an alterative impresses its like on the thing altered. And although, if we consider only the quantitative extremes, there is need in all cases for contact to be mutual, nevertheless if we consider action and passion, we shall find certain things to be touching only, and others only touched: since the heavenly bodies touch the elemental bodies in this way, in so far as they alter them; and yet they are not touched by them, since they do not suffer from them. Accordingly if there be any agents which are not in contact by their quantitative extremes, they will be said nevertheless to touch, in so far as they act, in which sense we say that a person who makes us sorrowful touches us. Wherefore it is possible for an intellectual substance to be united to a body by contact, by touching it in this way. For intellectual substances act on bodies and move them, since they are immaterial and more actual.

This contact however is not quantitative but virtual. Wherefore this contact differs from bodily contact in three ways. First, because by this contact the indivisible can touch the divisible. Now this cannot happen in bodily contact, because nothing but what is indivisible can be touched by a point. Whereas an intellectual substance, although indivisible, can touch a divisible quantity, in so far as it acts upon it. For a point is indivisible in one way, and an intellectual substance in another. A point is indivisible as being the term of a quantity, wherefore it has a determined position in a continuous quantity, beyond which it cannot stretch. But an intellectual substance is indivisible, as being outside the genus of quantity: so that no quantitative indivisible is assigned with which it can come into contact. Secondly, because quantitative contact is only in respect of extremes, whereas virtual contact regards the whole thing touched. For it is touched inasmuch as it suffers and is moved. Now this is according as it is in potentiality: and potentiality regards the whole and not the extremes of the whole. Wherefore the whole is touched. Whence follows the third difference. Because in quantitative contact which takes place in regard to the extremes, that which touches must be outside that which is touched, and cannot pierce it, since it is hindered by it. Whereas virtual contact, which applies to intellectual substances, since it reaches inwards, makes the touching substance to be within the thing touched, and to penetrate it without hindrance.

Accordingly an intellectual substance can be united to a body by virtual contact. Now things united by contact of this kind are not one simply. For they are one in action and passion, which is not to be one simply. For one is predicated in the same way as being. But to be an agent does not signify being simply. Consequently neither is to be one in action to be one simply.

Now one simply is taken in three ways: either as being indivisible, or as being continuous, or as being logically one. But the one which is indivisible cannot result from an intellectual substance and a body: for the one of this latter kind must needs be composed of the two. Nor again can the one that is continuous, because the parts of the continuous are parts of quantity. It remains therefore for us to inquire whether from an intellectual substance and a body can be formed the one which is one logically.

Now from two things that stay there does not result something logically one, except from substantial form and matter: since from subject and accident there does not result one logically, for the idea of man is not the same as the idea of white. Hence it remains for us to inquire whether an intellectual substance can be the substantial form of a body. And to those who consider the question reasonably it would seem that this is impossible.

For from two actually existing substances there cannot be made something one: because the act of a thing is that whereby it is distinguished from another. Now an intellectual substance is an actually existing substance, as is clear from what has been said and so likewise is a body. Therefore, seemingly, something one cannot be made from an intellectual substance and a body.

Again. Form and matter are contained in the same genus: since every genus is divided into act and potentiality. But intellectual substance and body are of different genera. Therefore it does not seem possible that one be the form of the other.

Moreover. Everything whose being is in matter must be material. Now if an intellectual substance is the form of a body, its being must be in corporeal matter, since the being of the form is not beside the being of the matter. Hence it will follow that an intellectual substance is not immaterial, as above it was proved to be.

Again. That which has its being in a body cannot possibly be separated from that body. Now it is proved by philosophers that the intellect is separate from the body, and that it is neither a body nor a power in a body. Therefore an intellectual substance is not the form of a body, for thus its being would be in a body.

Further. That which has its being in common with a body, must have its operation in common with a body, because a thing acts inasmuch as it is a being: nor can the active power of a thing surpass its essence, since power results from the essential principles. But if an intellectual substance be the form of a body, its being must be common to it and the body: because from form and matter there results one thing simply, that exists by one being. Consequently an intellectual substance will have its operation in common with the body, and its power will be a power in a body: which has been proved to be impossible.