The operations of any being follow from its nature...

The operations of any being follow from its nature; thus any form leading only to organic operations is bound to matter and follows the conditions of matter, as, for instance, the animal soul, which is corrupted with the organism. But the human soul has superorganic operations. The intellect does not need any organ in its understanding; hence the human soul is a superorganic substance, not dependent for its being upon any matter.

And despite the fact that the human soul is the form of the body, it will last as a separate substance of intellectual nature, even when the conditions of the body render impossible the functioning of the soul as the form of the body.

4 Thus the doctrine of Aquinas concerning the soul in general and the human soul in particular, may be summed us as follows: When the form in matter is the origin of immanent actions, it gives origin to life and as such is more particularly called the "soul." There is a vegetative soul, such as the principle of plants, whose activity is fulfilled in nutrition, growth, and reproduction.

Superior to the vegetative is the sensitive soul, which is present in animals; besides the processes of nutrition, growth and reproduction the sensitive soul is capable of sensitive knowledge and appetition. Superior still to the sensitive soul is the rational soul. The rational soul is created directly by God; it is distinct for each man; it is the true form of the body.

The human soul performs the functions of the vegetative and sensitive life, but besides these functions it has activities which do not depend upon the body, i.e., understanding and volition. The intellect and the will are the faculties of the soul, the means through which it operates. The intellect has for its object the knowledge of the universal, and operates by judging and reasoning. The will is free; that is, it is not determined by any particular good, but it determines itself.

From an analysis of the intellect and the will, Aquinas proves the spirituality, the simplicity, and the immortality of the soul. The intellect has, in fact, for its proportionate object the universal, the understanding of which is a simple and spiritual act. Hence the soul from which the act of understanding proceeds is itself simple and spiritual. Since it is simple and spiritual, it is by nature also immortal.

The same conclusion is reached through an analysis of the will, which, as we have said, is free, i.e., not determined by any cause outside itself.