Aristotle was a pupil of Plato and author of numerous works on logic, the natural world and human society. He approaches poetry (drama) not as a literary critic but as an analytical philosopher. Poetics is credited as the source of the Aristotelian doctrine of the three unities - time, place and action - but in fact insists on only one of them. Much of its analysis of tragedy is questionable, though the idea of catharsis, of being purged by our experience of tragedy, has taken hold of the modern mind.