Democritus

Democritus, working with his teacher Leucippus, offered the most prescient answer in pre-Socratic philosophy: the world is made of indivisible particles (atoms — atomon means "uncuttable") moving through empty space (the void). Everything else — color, taste, the feel of warmth — is convention. Atoms and void are real; the rest is how atoms interact with us.

This is recognizably the picture modern physics will rediscover two thousand years later. Democritus did not arrive at it through experiment. He arrived at it through argument: if change is real (against Parmenides) but matter is conserved, then what changes must be the arrangement of indestructible building blocks.

Plato reportedly wanted Democritus's books burned. Aristotle disagreed with him but treated him as a serious rival. Lucretius and Epicurus made his atomism the foundation of an entire ethical philosophy: in a universe of atoms, there are no gods watching, so be unafraid.