St. Thomas Aquinas explains his statement, "the soul is man," in two senses or ways. One is that a man is composed of not just the body (or form) but also of the soul. Some suggest that "form alone belongs to the species, while matter is part of the individual, and not the species." Aquinas denies this. He maintains that the nature of a particular species is what its definition signifies, and natural things are defined according to both their form and matter. Matter, according to him, is part of a particular species and shared by others under the same species: a particular man is composed of a soul, flesh and bones like other men or individuals in common. What belongs to the substance of a particular man also belongs to the substance of other individuals belonging to the same given species (Aquinas 1947).

Another way the statement can be understood is in terms of the operation of the sensitive operations of the soul. If the operations of the sensitive soul of this man were proper to it, as all such operations as attributable to man, then whatever performs those operations proper to that soul is that thing. In other words, if what this sensitive soul does is what is proper to it, apart from the body or form - because all such actions or functions, which man does, are performed by the soul only - then that which performs the acts proper only to a man is a man, not a tree or a stone.

But the capacity to sense is an operation not only of the soul but also of the body. Sensation, Aquinas says, is an operation performed by man, but is not proper to him (only). Animals and plants can sense, too.

He clarifies between the "inward" man and the "outward" man. In man, there is an intellectual part, which is in accordance with truth, and this he refers to as the "inward" man. The sensitive part of man he calls the "outward" man. This "outward" man is what those whose observations of life do not exceed the physical senses call "man."

He refers to the hypostasis of the soul and body in the human species. This hypostasis does not refer to every particular substance, such as a hand...

The Divine Nature is a pure act that receives or needs nothing from any other and "admits of no variety in itself." In the meantime, the human soul remains in a state of potentiality when in the exercise of intelligence. It can acquire knowledge only from things and possesses various powers incompatible with those of the Divine Nature. Aquinas perceives this error in thinking as originating from two ancient statements. One could not rise above the imagination and assumed that only bodies existed. Hence, this group of men believed that God was a body, but that God's body was the principle of other bodies. These men thought that the soul had the same nature as the body, which to them was the first principle. This is the claim made in De Anima (Aristotle 1993) and concluded that the human soul had the same nature as God's.
The other group consisted of the Manichaeans, who believed that God was corporeal or bodily light. They held that the human soul was part of that light and bound up with the human body. The belief advanced into suggesting the existence of something non-bodily, not apart from the human body, but the form of a body. Hence, their representative uttered that "God is a soul governing the world by movement and reason.

This conclusion led them to suppose that man's soul was part of the one governing soul and that man is a part of the whole world. But these proponents could not understand the different degrees of spiritual substance as the distinction of bodies. But Aquinas says that all these theories are false, because the human soul is not of the same substance as God.

Aquinas retains Origen's supposition that the souls of all men, from the first, were created at the same time with the angels and that both were equal in their natural condition. They differed only according to merit and some of them had bodies, like those of men, while others remained free of matter.…