On the Soul is divided into three books. The first consists mainly of a review of the opinions of Aristotle’s predecessors about the soul, and refutation of their errors. The second book and the first part of the third define the soul and describe and explain the nutritive and sensitive faculties. The rest of the third book treats the intellect.
The original meaning of the word psych was “breath,” and in the earliest Greek literature it had come to stand for “breath-soul,” being identified with vital functions in general, while a separate blood-soul was held to be the seat of consciousness. Aristotle seems to have been unaware of this view. By his time, psych meant “life-principle,” whether simple or complex, the inner cause of vital movements of all kinds.