On the Soul (De Anima)

A treatise in three books on the soul, which for Aristotle means whatever makes a living thing alive. Book I reviews earlier views, from the atomists to the idea that the soul is a harmony, and finds them wanting. Book II gives his own definition: the soul is the first actuality of a natural body that has life potentially, the form of the living body. There is no more need to ask whether soul and body are one than to ask this of wax and the shape stamped on it. Souls are ranked by capacities: nutrition and reproduction in plants, perception and movement in animals, and thought in humans. He then treats each of the five senses, and describes perception as taking on the form of an object without its matter, as wax takes a seal's impression. Book III covers imagination, desire and the intellect, including a famously obscure chapter on an intellect that is separable and undying.

It offers a third way between dualism and materialism that many philosophers of mind find attractive today, and it founded psychology as a study of the capacities of living things.