Atoms and Void
Leucippus and Democritus made the boldest move available. Parmenides had argued that the void, what-is-not, cannot be, and that motion therefore cannot occur. The atomists replied that what-is-not exists.
That sounds like nonsense until you see what it means: the void is a real feature of the world, an emptiness that genuinely obtains. Once granted, everything follows. Reality consists of atoms — indivisible, eternal, unchanging bits of being, satisfying Parmenides completely — moving in void, which supplies the emptiness motion requires.
Atoms differ only in shape, size, arrangement and position. They have no colour, no taste, no temperature, because those are not properties of atoms at all. Democritus draws the conclusion explicitly: "By convention sweet, by convention bitter; in reality atoms and void." This is the first clear statement of the distinction later called primary and secondary qualities: the shape of a thing is in it, the sweetness is in the interaction with us.
The combinatorial power is what makes the theory attractive. A small number of atom types, arranged differently, generates unlimited variety, exactly as a small alphabet generates unlimited text. Democritus made the analogy himself.
The universe is infinite and contains innumerable worlds, some like ours, some not, forming and dissolving continuously. There is no purpose anywhere: atoms move by necessity, and order arises without anyone intending it. This is the first thoroughly mechanistic and non-teleological cosmology, and it is why Aristotle, who explained by purposes, argued against it so hard, and why atomism nearly disappeared before its recovery in the seventeenth century.
Democritus also wrote extensively on ethics, recommending cheerfulness achieved through moderation, an unexpected but consistent conclusion, since in a universe with no purpose, well-being is a matter of arranging one's own life rather than aligning with a cosmic plan.
Common pitfall: treating ancient atomism as an early version of modern atomic theory. It was a metaphysical response to Parmenides, argued a priori, not an empirical hypothesis. Its atoms are conceptually indivisible, that is what "atom" means, where modern atoms are divisible and were found by experiment.