Epicurus, answered from the texts and cited to the page.
The universe consists of two things: atoms and void. Atoms are bodies that cannot be split — uncuttables — possessing size, shape, weight, and resistance-to-blows but no other irreducible properties. Void is empty space through which atoms move. Macroscopic bodies are conglomerations of atoms; their sensible qualities (color, taste, warmth) emerge from the configurations and motions of the underlying atoms but are real for the compounds.
Atoms fall through void at uniform velocity under their own weight; at random intervals they undergo a tiny indeterministic deviation — the clinamen, the parenklisis, the swerve — which both initiates the collisions out of which all macroscopic bodies form and grounds the possibility of free action in a universe that would otherwise be wholly determined.
The soul is material, composed of atoms of a particularly fine and quick-moving kind distributed throughout the body and concentrated in the chest. At death the soul-atoms disperse and the person ceases to exist. The world is one of infinitely many worlds, none created by gods; in the infinite stretches of time and space, every possible configuration of atoms occurs eventually, including worlds like ours and worlds nothing like ours.123
And so in each shape the atoms are quite infinite in number, but their differences of shape are not quite infinite, but only incomprehensible in number. 43 And the atoms move continuously for all time, some o them (falling straight down, others swerving, and others recoiling from their collisions.The Extant Remains, pp. 25–27
Similarly the mouth's taste is separate, and odours come to be separately, and sounds too are separate. And so it is necessary that one set of senses not be able to refute another. Nor, moreover, will they be able to criticize themselves, since they will at all times have to command equal confidence. 28: On the Nature of Things 2.216-293 excerpts.The testimony of Lucretius, p. 66
Since a great deal of matter was still contained in the earth and this was packed densely by the blows of the [atomic] bodies and by those from the rays of the heavenly bodies, [the earth's] entire configuration, which was made up of small particles, was squeezed together and [so] produced the nature of fluids.Short fragments and testimonia from uncertain works, pp. 93–95
The foundation of the entire Epicurean system. Modern science has falsified many of the specific empirical claims: atoms in our sense are not indivisible (they have substructure), atoms do not fall straight downward at uniform velocity (gravity is general attraction, not preferred direction), the mind is not located in the chest, sensible qualities like bitter taste are not produced by rough particles tearing at the tongue. Tim O'Keefe's Epicureanism (2010) gives the standard contemporary acknowledgment of these falsifications. The seventeenth-century scientific revolution (Gassendi, Boyle, Newton) revived a modified atomism explicitly indebted to Epicurus; Catherine Wilson's Epicureanism at the Origins of Modernity (2008) tracks the early-modern reception. The broader naturalistic worldview — no purpose in nature, no providence, death as annihilation, freedom in a material universe — is widely held in contemporary philosophy and science even where the specific Epicurean physics has been replaced.