Aristotle (384-322 BC) was a Greek philosopher and scientist who had a major role in point of divergence of Plato Succeeds.
The point of divergence.
Plato' life was the same as the OTL. Aristotle was behind the POD, apparently thanks to a stroke of fortune. It is said that while reading the Republic, the philosopher went on a walk. During this he turned left instead of right and as a result was in the path of a loose and rampaging wild horse. Fortunately, a priestess of Athene saved him, but was killed in the process. Impressed, Aristole develops a positive view of women as prescribed by Plato. Although making his own amendments, Aristotle became an advocate for the Kallipolis. It is not known if this story is true. Some claim that the reason Aristotle turned was simply after being defeated in an argument with Plato's supporters and was thus sufficiently convinced. While democracy becomes more appealing and the form of the good is replaced by virtue ethics, Plato's system is preserved more intact by Aristotle, who influenced Alexander to make a similar empire. This became the Kallipolic Confederation.
However Aristotle made several amendments based on his own studies. Yes, women are equal, but they are still weaker and this should be noted. Also, while Plato discouraged marriage in the Republic, he supported it in his later work, suggesting that being unmarried is antisocial. Aristotle clarified it to mean it was antisocial for Civilians to be single, whereas the Guardians don't have to be. For the non-combatants, marriage was the highest form of friendship. Even among the military where such behaviour is discouraged, unions may still form. In some countries, such as Britain and Rome, both the military and workers may marry (some even cross classes, if this happens the Guardian outranks the Civilian which led to some Roman families having a materfamilias rather than paterfamilias). Aristotle also held a more positive view of democracy than Plato, believing it the second highest form of government (the highest being polity, where decisions are based on how they affect everyone rather than based on whose mob is larger) and proposed democracy in the sub-governments of Kallipoli. Of course, the navigator of the ship of state (who knows where they're going) and the captain (the ruler) must make the final decision, but the crew should not be ignored. The engineers, helmsmen, scientists, marines and (possibly) rowers are also vital.
Aristotle believed that there was no such thing as atoms, instead everything is made of a elements: earth, fire, water, air and ether. He was also a proponent of the four humours theory in medicine. Much of Aristotle's scientific work was later disproved, but his influence remains. For example, he believed in the moral system of virtue ethics rather than the form of the good of Plato, pointing out that the forms he talks about (perfect versions of everything in existence, which exist in a world of their own) cannot be proven. However he claimed that barbarians cannot achive virtue ethics' ultimate goal eudaimonia as they lack the intellectual capacity of the civilised i.e. Greek people and had less free time to consider so couldn't reach the highest state possible. Of course, it may be possible, for barbarian nations to be educated into civilization. By the Middle Ages, it was agreed that Western Europe, South Asia and North Africa were no longer barbaric.