Parmenides

Parmenides of Elea, a Greek colony in southern Italy, founded the Eleatic school and set out his thought in a hexameter poem of which substantial fragments survive. In it a goddess reveals two ways of inquiry. The Way of Truth argues that what is, is, and cannot not be: Being is one, whole, ungenerated, imperishable, and unchanging, since any coming-to-be or passing-away would require what-is-not, and what-is-not can be neither thought nor spoken. The Way of Opinion then describes the deceptive, plural world of appearances that mortals trust, complete with a cosmology — included, the goddess says, so that no mortal account shall outstrip the hearer.

His most radical claim is that thinking and being are the same: only what can be is can be thought, so genuine reality must be exactly as reason requires — single, motionless, and full. Change, plurality, and the void are illusions of the senses. This argument forced every later thinker to explain how a world of apparent motion and multiplicity could be reconciled with the demands of reason, and it stands behind Plato's distinction between the changeless Forms and the flux of the sensible world.

Plato honored him as no other predecessor, naming a dialogue after him in which the aged Parmenides gently interrogates a young Socrates about the theory of Forms. The Neoplatonists read that dialogue as a coded map of all reality, and took Parmenides' One as a foreshadowing of the One beyond being — making the austere Eleatic, against all likelihood, a patron of their metaphysics.