Démocrite d'Abdère
Nothing exists but atoms and the void.
Misconceptions
Democritus laughed at everything: he was "the laughing philosopher".
The image is late: it appears in Roman authors, more than three centuries after his death (the poet Horace, outside our corpus, then Seneca c. 50-60 AD). In On Anger he writes that Democritus "is said never to have appeared in public without laughing": it is a rumour he reports. In On Tranquillity of Mind he makes him a model, set against the weeping Heraclitus: better to laugh at human vices than to lament them. None of the fragments we have and no passage of Diogenes Laertius speaks of laughter. What Diogenes Laertius reports is a doctrine: the aim is euthymia, a calm and steady soul, which must not be confused with pleasure. Even later texts develop the image, such as forged letters attributed to the physician Hippocrates, or Lucian (2nd c. AD), who stages the laughing Democritus against the weeping Heraclitus: we have not collected them.
Democritus invented the atom on his own.
Aristotle often speaks of "Leucippus and his companion Democritus" together, and Diogenes Laertius notes that Theophrastus' school attributed to Leucippus his main book, the Great Diacosmos. We know very little about Leucippus, so his exact role cannot be fixed. It is safer to say that atomism is a theory born in the Abdera circle and developed by Democritus.
Democritus' atoms are already those of modern physics.
Only the word is the same (atomos, "uncuttable"). For Democritus and Leucippus, according to Aristotle, the elements are the full and the void, that is, being and non-being, and atoms differ only in shape, arrangement and position, like the letters A and N, or AN and NA. They are eternal and solid. Today's atoms have parts, charges and energies, and can be split. Above all, Democritus did not prove his idea by experiment: it is a piece of reasoning, which Aristotle judges careless about the origin of motion.
Plato hated Democritus and wanted to burn his books.
The story comes from Diogenes Laertius (c. 230 AD), who cites Aristoxenus (4th c. BC): Plato supposedly wanted to burn all the writings of Democritus he could collect, but Pythagoreans talked him out of it because they were already everywhere. Diogenes adds a clue: Plato, who speaks of nearly all the early thinkers, never mentions Democritus. But this is an anecdote reported from afar. A silence can be explained otherwise, and we have no text by Plato about him.
Democritus put out his own eyes to think better.
Cicero (Tusculan Disputations V, c. 45 BC) writes only that Democritus, having lost his sight, could no longer tell white from black but could tell good from evil, and that he thought sight hinders the mind. In this passage he does not say he blinded himself. Elsewhere (On Ends V, outside our corpus), Cicero does report the rumour that he deprived himself of sight, but adds that he does not know whether it is true. Other, later authors repeat the story. It is an anecdote, not an established fact.