St. Thomas asks, Whether the soul is composed of matter and form? (ST I., Q.75, A.5.)
His answer is no, and he demonstrates it in two ways:
First, on the ground that the soul is defined as the form of the body, as opposed to the matter of the body. He had previously said that “the soul, which is the first principle of life, is not a body, but the act of a body”. (ST I., Q.75, A.4.) The soul is what the body does. When the body stops acting — stops breathing, eating, sleeping — we say the soul has left it.
For this reason the soul is analogous to act, and matter is analogous to potency. Matter is in potency to any number of forms. For example carbon can take the form of coal, or it can be part of oil or gasoline, or part of a carbon-based life form, which in turn can be anything from a single-celled organism to a plant to a man. But the form of a man can only be the form of a man; it is not in potency to any other form. It is the soul of a man which makes the matter of which he is composed to become a man, as opposed to a lifeless heap of matter, which is what a dead body is. The soul is the act of man-ness, to which the matter composing the body is only in potency, until such time as the soul actualizes it, that is, makes it actually a man.
Second, with regard to the specific case of the human soul, on the ground that if the intellectual soul were made of matter, then it could only know individual things and not universal things. Consider by way of illustration the senses of the human body: First, they can only sense physical things; and second, they can only “know” the specific thing that they are sensing at any given time. That is, they can’t know universal things. They can sense this dog, but have no way of sensing universal dog-ness.
St. Thomas explains that “whatever is received into something is received according to the mode of the recipient”. Physical sensations are received according to the mode of the sense which is doing the sensing. Thus, a dog is received into the eye only insofar as it reflects light. It is received into the sense of touch only insofar as it has a surface which may be touched. Same with the other senses.
But what happens when we think of or have knowledge of the concept of dog-ness? In that case the whole dog enters our mind at once, not merely its appearance or texture or smell or sound, but all of it together. And not merely a particular dog, but the essence of dog-ness, by which we recognize any number of physical animals as being the same thing. This is not an individual dog with a specific appearance; it can be either black or white, large or small, longhaired or shorthaired; therefore it’s not a physical thing, since every physical thing has one specific set of physical attributes. A single physical dog is either white or black, but not both, large or small but not both, longhaired or shorthaired, but not both. Whereas the concept of dog-ness allows for variations in all of those attributes as part of the very concept itself.
Because dog-ness has no specific physical attributes, it is non-physical. And if “whatever is received into something is received according to the mode of the recipient”, then whatever it is that receives the concept of dog-ness, which we call the intellect, must itself be non-physical.