One important distinction within the natural world is that some natural substances are alive and others inanimate. The former are distinguished from the latter by their possession of a soul or animator. ‘Psychology’ explains that Aristotle’s souls are not pieces of living things or bits of spiritual stuff inside physical bodies; rather, they are sets of powers or capacities. Souls are fulfilments of bodies and cannot exist independently. Aristotle’s treatment of thought is both obscure in itself and hard to reconcile with the rest of his psychology. However, that should not detract from his work on psychology, which is persistently scientific in its approach.