The philosophers Protagoras, who lives from about 485 BCE to 410 BCE, and Democritus, who lives from about 460 BCEto about 370 BCE) are among the famous citizens of Abdera (present Ávdhira, Greece), a prosperous member of the Delian League in the fifth century.
Crippled early in the fourth century BCE by Thracian incursions, the city declines sharply in importance.
Democritus develops and systematizes the theory of classical atomism.
The theory, credited to his teacher Leucippus, postulates a world composed of hard, indivisible (hence atomic, from Greek atoma, "uncuttable") particles of matter traveling through empty space.
Atoms, in Democritus’ view, have various shapes (“why,” he asks, “ should they have one shape rather than another?"), mass, and motion; but such subjective qualities as color or flavor are supplied by the observer.
He describes atoms as existing by convention or by custom (nomos), as opposed to existing by nature (physis), and explains all change by reference to the transfer of momentum as the atoms collided.
Democritus theorizes that our cosmos was formed by a spinning vortex of such atoms and that there are an infinite number of worlds created in the same way.
In his belief in the unchanging nature of the intelligible universe and the changing nature of the sensible universe, he directly opposes the ideas of his fifth-century predecessors Heraclitus, who denied all constancy, and Parmenides, who denied all change.
Democritus' ethical naturalism, positioned between the extremes of these two philosophers, rejects any teleology or belief in chance that would deny people's responsibility for their own well-being.
Arguing that it is an individual's conscience alone that determines right or wrong action, he rejects all supernatural sanctions of human behavior, and maintains that belief in an afterlife is a ridiculous fiction. (This kind of thinking earned Democritus the label of the "laughing philosopher," in contrast to Heraclitus, the "weeping philosopher.")
