Aquinas’ first task is to define what the soul is. First, he distinguishes the soul from the body. Some ancient philosophers considered the soul to be a body. Aquinas asserts that it is “the first principle of life” but is distinct from the body. The soul is what distinguishes “animate” from “inanimate” beings.

In addition to being incorporeal, the soul is subsistent—i.e., it has an operation of its own. However, this does not mean that the soul leads an existence separate from the body. In fact, it requires the body for its operation, especially in receiving sense knowledge. In contrast to non-human animals, human beings have souls that are intellective. In fact, Aquinas defines the soul as the “intellectual principle” in man, but also stresses that man is a body-soul composite; the soul is not the whole of a human being.

The human soul is incorruptible. This is because as a subsistent form, being belongs to it by its very nature. The human soul has a natural desire for being, and a natural desire can never be frustrated. Thus, when death occurs, the soul is separated from the body but does not cease to exist.