139. Psychology is the study of living things (psuchê, the vital principle). Now, life consists in the power of immanent action -- i.e., in action that affects the agent alone its effect is self-evolution and self-perfection.
There are three degrees of life: vegetative, sensitive, and intellectual life. These rise in dignity above one another as they become more independent of matter:
1. Vegetative life needs material organs and assumes into itself material food; still, it is so far above mere matter that it controls the physical and chemical powers of such food, which it converts into its own substance.
2. Sensitive life also needs material organs, and perceives only material things; but it assumes into itself the images only of the objects, not the gross matter, in its action of sense-perception.
3. Intellectual life, as such, needs no material organs, nor even material images: the soul can understand things entirely immaterial, and material things in an immaterial manner, though, as long as it is substantially one being with the body, it understands its objects in connection with bodily Phantasms. We distinguish, therefore, all living creatures to which our natural knowledge extends into three classes or genera, viz.: plants, which have only vegetable life; animals, which have vegetable and sensitive life; and man, who has vegetable, sensitive, and intellectual life.
We shall consider: 1. The specific nature of plants and animals. 2. Sensitive and rational cognition. 3. Sensitive and rational appetite. 4. The nature of the human soul. 5. The origin and the destiny of the human soul.