Aristotle (384–322 BCE) entered Plato's Academy at seventeen and stayed twenty years. After Plato's death he left Athens, lived for a time in Asia Minor, and served as private tutor to the boy Alexander, son of King Philip of Macedon. Returning to Athens in 335 BCE, he founded his own school in a public grove called the Lyceum. Because he is said to have taught while walking, his followers came to be called *Peripatetics* — 'those who walk about'.

The Lyceum was less a school than a research institute. There Aristotle and his students did something no other ancient institution did: they partitioned knowledge into fields, and gave each field its own method. Logic in the *Organon*, natural philosophy in the *Physics* and *On the Heavens*, systematic zoology in the *History of Animals*, the study of being qua being in the *Metaphysics*, the anatomy of moral life in the *Nicomachean Ethics*, the comparison of hundreds of city-state constitutions in the *Politics*. The method is always the same: observe, classify, define, prove.

Philosophically he breaks decisively with Plato. The Forms do not lie in a separate realm; they are *in* individual things. A horse is a horse not by reference to an external 'Form of Horse' but through the relation between its own form and its matter. Knowledge is not heavenly recollection but the induction of universals from sensory experience. Ethics, similarly, is the lived practice of building virtuous character into a happy life (*eudaimonia*); the mean between extremes is not an abstract rule but a judgement made well in context.

After Alexander's death Aristotle was charged with impiety, as Socrates had been; he is said to have left Athens 'so that the city would not sin twice against philosophy.' His works passed into the medieval Islamic world through Averroes and Avicenna, then into Latin Christendom through Aquinas, and the modern scientific revolution began by arguing with him. The division of knowledge into separately practised fields — biology, physics, logic, ethics — which still structures every university, is the Lyceum's legacy.