De AnimaAristotle

Aristotle's De Anima asks what it means for something to be alive. The soul (psyche) is not a separate substance trapped in a body. It is the form of a natural body that has life potentially. A body without a soul is a corpse; a soul without a body is (with one possible exception) nothing at all.

Book I surveys previous theories and finds them wanting. Those who make the soul a harmony, a self-moving number, or a blend of elements all fail to explain how soul and body form a genuine unity. Book II introduces Aristotle's own account through a hierarchy of soul-functions. Plants have nutritive soul (growth, reproduction). Animals add perception and locomotion. Humans add intellect. Each higher capacity presupposes the lower, the way a square presupposes a triangle.

The treatment of perception is detailed. Each sense has a proper object (color for sight, sound for hearing), and perceiving consists in receiving the form of the object without its matter, as wax receives the impression of a signet ring. Imagination and memory are explained as residual movements left by perception.

Book III introduces the intellect (nous), and here Aristotle is at his most compressed and contested. The active intellect, which "makes all things," is separable, impassible, and immortal. Whether this is a part of the individual human soul or something divine operating in us has been debated from Alexander of Aphrodisias through Aquinas and beyond.