The Academy in Athens taught that the visible world is the lesser one. Real knowledge was knowledge of Forms — perfect, unchanging, located nowhere in particular — and the horse in the field a smudged copy of something reachable only by argument. A physician's son from the Macedonian coast arrived there at seventeen and stayed twenty years, and by the end of them he had decided his teacher had it the wrong way round. The horse is the real thing. Whatever deserves to be called its form is inside it, and the way to reach it is to look.
That is the claim to carry away: things are found out by examination, not by reasoning past them. It sounds obvious now, which is a measure of how far it travelled.
He did not reach it in comfort: when Plato died in 347 the school passed to his nephew Speusippus, an Athenian citizen, while Aristotle was a resident foreigner with no political rights in the city and no claim on it. He left. What followed were the years that made him — Assos on the Anatolian coast, then two years on Lesbos, questioning fishermen and cutting things open on the lagoon at Pyrrha. He described a modified arm by which the male octopus transfers sperm; naturalists treated it as a fisherman's tale for two thousand years, until zoologists confirmed it in the nineteenth century. He was also wrong, at length: he put thought in the heart and made the brain an organ for cooling blood.
Out of that habit of dissection came a method. To understand anything, he argued, four different questions have to be answered about it — what it is made of, what shape or structure makes it that rather than something else, what set it going, and what it is for. The last question carried his whole system, and he asked it of human beings too. If a knife is for cutting, what is a person for? His answer, worked out in the Nicomachean Ethics, is that a life goes well when it exercises reason well, over a whole life, in company. Virtue comes by repetition rather than by knowledge: one grows just by doing just things, the way one grows into a builder by building. And he refused to make it exact: ethics, he insisted, admits only the precision its subject allows, and demanding proofs from a moral argument is as crude as accepting guesses from a mathematician.
He tutored the young Alexander from 343, then returned to Athens and opened a school of his own around 335. The surviving works are the working papers of that school — compressed, argumentative, repetitive, plainly meant to be talked through rather than published. The polished dialogues he did release, admired in antiquity for their style, are lost. What survives was sorted into the corpus still in use by Andronicus of Rhodes in the first century BCE; Metaphysics is not his title but a later one: the treatises that came after the Physics, though whether that means on the shelf or in the order of study is disputed. How the texts got from Athens to Andronicus is disputed and probably always will be.
Alexander died in 323 and Athens turned on its Macedonians. Aristotle was charged with impiety over a poem he had written for Hermias of Atarneus, his patron and father-in-law, who had been tortured to death by the Persian king. He left for Chalcis and died within the year.
The afterlife was stranger than any of it: his natural philosophy was banned at the University of Paris in 1210 on pain of excommunication; sixty years later he was simply the Philosopher, cited that way and no other, and in 1277 the bishop of Paris condemned 219 propositions in an attempt to contain him. His physics was demolished in the seventeenth century, and that demolition is roughly what the phrase scientific revolution names. His biology held up better than anyone expected. His ethics came back from the dead in 1958, when Elizabeth Anscombe argued that modern moral philosophy had been talking nonsense since it abandoned him, and again in 1981 with Alasdair MacIntyre. Virtue ethics is now one of the three positions every student is taught, and it is his.