Plato

Plato was Socrates' devoted student, and after Socrates' execution he spent the rest of his life writing dialogues in which Socrates speaks. He founded the Academy outside Athens, the first institution we would recognize as a university; it would run for nearly 900 years.

His central conviction is that the world we see is not the world that is. Beyond appearances lie eternal, unchanging Forms — the perfect Triangle of which every drawn triangle is a copy, the Good of which every good act is an echo. To know is not to gather sense-data; it is to ascend, through reasoning, from the cave of shadows into the sunlight of the Forms.

He wrote about everything: love (Symposium), death (Phaedo), justice (Republic), language (Cratylus), beauty (Phaedrus). Whitehead said all of Western philosophy is "a series of footnotes to Plato." This is only slightly an exaggeration.