Nothing exists except atoms and empty space; everything else is opinion.

Commonly presented as a summary of Democritus’s atomist view: reality is fundamentally composed of indivisible particles moving in void, while many everyday qualities are treated as dependent on perception or convention.

It draws a line between what is taken to be objectively real (basic physical constituents and the void) and what is regarded as secondary or mind-dependent (many judgments and perceived qualities), urging skepticism about treating appearances and beliefs as ultimate reality.