In the preceding sections, we have demonstrated that human possesses an immaterial intellect and a free will. The intellect is not human, nor is the will: both are powers through which human operates. Technically we call them faculties (accidents) – that, the immediate principles of mental operations. Since these faculties are not the personal self, they must exist in the person. But since they are immaterial, they cannot exist in the person’s body, which is material. Hence there must be in the human person, besides one’s body, some other component in which one’s spiritual faculties inhere. We call this component the soul. Therefore our first provisional definition of the soul might be that component of human in which the intellect and the will inhere; or, that component which manifests itself in human’s thinking and willing.
Human Soul is Spiritual /immaterial
Spiritual means are intrinsically independent of matter. A being is spiritual if it does not require matter as co-cause of its operations and of its existence. But we have demonstrated that the intellect and the will, through which the soul acts, are intrinsically independent of matter. But as a being is, so it acts. Therefore the human soul is also intrinsically independent of matter, it is spiritual.
Human Soul is Simple
The human soul is simple in the sense that it is not composed of matter. There are no substance and accidents in the soul. Being immaterial, the human soul is not only undivided but also indivisible. The human soul is not only a substance that which exists in itself but also a subsistence. Because it exists by itself. The human body and other material things are not the substratum that supports and sustains the human soul in existence.
Human Soul is Immortal
Immortal means not subject to death or to destruction, destined to exist forever. A being’s essence can be destroyed directly by decomposition, indirectly by loss of essential support. A being’s existence is destroyed by annihilation. The human soul cannot be destroyed by decomposition. Only composite, or material, beings can be decomposed. The human soul cannot also be destroyed by the loss of essential support. There is the loss of essential support when a being which is intrinsically dependent on matter for its operations and for its existence loses the support of that matter. For our soul is not intrinsically dependent on matter for its specific operations. Therefore it cannot be destroyed by the loss of essential support. The third way in which a being can be destroyed is by annihilation.
Since God creates the soul as a being destined to exist forever. His will is immutable. Therefore He does not annihilate a being which he creates as a being destined to exist forever. Therefore the human soul is immortal.