Leading philosopher and man of science, pupil of his compatriot Leucippos; known under the surname Gelasinos (the jovial one), he is considered the father of atomic theory. According to his definition, atoms are the ultimate molecules of matter which can neither be split or expanded/reduced, they are unborn, indestructible, incorruptible and indivisible, complete and perfect, compact, uniform and simple, infinite in number, variety and form, and in constant motion in a vacuum. Democritos was the author of dozens of treatises, but only their titles and brief references to them in works by later writers have survived.