52. Psychology. -- Psychology as a distinct science owes its origin to Aristotle; for he did not subordinate his study of man to a general understanding of the world as his predecessors had done: he employed the method proper to psychology -- internal and external observation and reasoning. Even at the present day Aristotle's researches on psychology retain their value.

The soul is the first act (entelechy) of a natural body, i.e., a body potentially possessing life, hê psuchê estin entelecheia hê prôtê sômatos phusikou (Treatise on the Soul, ii., 1); it is the substantial form of the living thing, as the body is the primary matter of the latter. Since, then, every living being possesses a soul, we might distinguish animal and vegetative psychology from human; but, as in the hierarchy of beings the higher species have the perfections of the lower, a complete study of man will take in life under all its aspects.

The soul, though fundamentally one, manifests itself by different faculties. Aristotle has not explained himself clearly on the distinction between them, but he seems to have considered them as different aspects of the same reality, the soul. The question is one that was to be studied more fully in the Middle Ages. The following are the main problems relating to the activities and to the nature of the soul.