Epicurus summarizes the key doctrines from “On Nature” (of which only a few fragments have been recovered) in this letter to Herodotus. At a time when most philosophers still endorsed what amounted to a magical world-view, Epicurus squarely places himself in the atomistic/materialistic tradition of Democritus. Epicurus also adds some brilliant and daring insights of his own, notably in his explanation of mental function in terms of the movements of specialized neural atoms and his suggestion that the universe is filled with other worlds where extraterrestrial life is possible.