Greek philosopher (c. 460–c. 370 BC)
Democritus (c. 460 BC – c. 370 BC) was a pre-Socratic Greek philosopher. He is popularly known as "the laughing philosopher" for advocating a cheerful outlook, and for his rhetorical use of irony and ridicule. A pupil of Leucippus, he was an influential pre-Socratic philosopher who formulated an atomic theory of the universe. Of his voluminous writings, only a few fragments of his ethical theory remain, with descriptions by other writers of his atomic theory.
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[Democritus says:] By convention sweet is sweet, by convention bitter is bitter, by convention hot is hot, by convention cold is cold, by convention color is color. But in reality there are atoms and the void. That is, the objects of sense are supposed to be real and it is customary to regard them as such, but in truth they are not. Only the atoms and the void are real.