From Saint Wiki
CHAPTER LXV: THAT THE SOUL IS NOT A BODY
THERE were also others who wandered further from the truth, by asserting that the soul is a body. And although these had different and various opinions, it will suffice to refute them here in general.
For living things, since they are physical beings, are composed of matter and form. Now they are composed of a body, and of a soul which makes them actually living. Therefore one of these must be the form, and the other the matter. But the body cannot be the form, since the body is not in something else as its matter and subject. Therefore the soul is the form. Consequently it is not a body, since no form is a body.
Again. It is impossible for two bodies to coincide. Now the soul is not apart from the body while the latter lives. Therefore the soul is not a body.
Moreover. Every body is divisible. And whatever is divisible requires something to keep together and unite its parts. Consequently if the soul were a body, it would have something else to hold it together, and this yet more would be the soul: since we observe that when the soul departs the body perishes. And if this again be divisible, we must at length either come to something indivisible and incorporeal, which will be the soul, or we shall go on to infinity, which is impossible. Therefore the soul is not a body.
Again. As we proved above, and as it is proved in 8 Phys., every self mover is composed of two parts of which the one is mover, the other moved. Now an animal is a self-mover, and the mover therein is the soul, while the body is moved. Consequently the soul is an unmoved mover. But no body moves without being moved, as we proved above. Consequently the soul is not a body.
Further. It was proved above that intelligence cannot be the act of a body. But it is the act of a soul. Therefore, at least the intellective soul is not a body.
As to the arguments by which some have tried to prove that the soul is a body, it is easy to solve them. For they prove that the soul is a body from the son being like his father even in the accidents of the soul, notwithstanding that the son is begotten of his father by bodily detachment. Also because the soul suffers with the body. Also because it is separate from the body, and separation is between bodies that touch one another.
But against this it has been already stated that the bodily temperament is somewhat the cause of the soul's passions by way of a dispositive cause. Again, the soul does not suffer with the body except accidentally because, since it is the form of the body, it is moved accidentally through the body being moved. Also the soul is separate from the body, not as that which touches from that which is touched, but as form from matter: although there is a certain contact between the incorporeal and a body, as we have shown.
Moreover many men were moved to take up this position through believing that there is nothing that is not a body, being unable to outstrip their imagination which is only about bodies. Wherefore this opinion is put forward in the person of the foolish as saying of the soul (Wis. ii. 2): The breath in our nostrils is smoke, and speech a spark to move our heart.
|Source: St. Thomas Aquinas, The Summa Contra Gentiles, trans. by The English Dominican Fathers from the latest Leonine Edition, Benzinger Brothers: New York, 1924.|