Xenocrates

Xenocrates of Chalcedon led the Academy for a quarter century after Speusippus, from 339 BCE. Ancient anecdote made him a byword for gravity and self-control — slow but thorough, incorruptible, so chaste that even a famous courtesan failed to tempt him. Industrious and orderly, he is credited with the standard division of philosophy into physics, ethics, and logic that became canonical for later schools.

He defined the soul as a 'self-moving number', identified the Forms with numbers, and populated the cosmos with ranks of daimones — intermediate beings, some good and some malign — mediating between gods and mortals. He divided reality into three regions: the intelligible realm above the heavens, the sensible realm below the moon, and a mixed heavenly region between, each grasped by a corresponding mode of knowledge.

This taste for hierarchy, triadic structure, and systematic schematization prefigures the elaborate architecture of later Platonism, and his demonology fed directly into the Middle Platonist and Neoplatonist picture of a cosmos full of mediating spiritual powers.