Aristotle

Aristotle studied at Plato's Academy for twenty years, tutored Alexander the Great, and then founded his own school, the Lyceum. His mind was the most systematic in the ancient world. He more or less invented logic, biology, literary criticism, political science, and the formal study of ethics – and his framework for each of these disciplines defined them for two thousand years.

Where Plato looked upward to eternal Forms, Aristotle looked at this world. Forms exist, but they exist in things, not in some separate heaven. To understand a tree you study trees. To understand justice you study how just people act. His method is empirical and patient: gather examples, find patterns, distinguish causes, define essences.

His ethics gives us virtue as the mean between extremes: courage is the middle ground between cowardice and recklessness. His politics calls humans the "political animal." His four causes (material, formal, efficient, final) gave the medievals their entire vocabulary for explanation. To read Aristotle is to read the architecture beneath all of Western thought.