Democritus, answered from the texts and cited to the page.
Cosmic process is governed by anankē (necessity) in the lawful collision, combination, and motion of atoms, and by tychē (chance) in our naming of those outcomes whose mechanical causes we do not yet trace in detail.1 There is no craftsman, no providence, no final cause, no purpose — neither in the cosmos as a whole nor in any of its parts.
The order we observe is the lawful unfolding of atomic configurations in void; the apparent purposiveness of living things is the mechanical consequence of how the atoms of their bodies happen to be configured by the cosmogonic process. Leucippus's verbatim fragment L1 — "nothing happens in vain, but everything from reason and by necessity" — states the doctrine in its most concentrated form: reason (ek logou) here is the lawful intelligibility of mechanical causation, not the purpose of any agent.2
Aristotle's polemic in Physics II, which characterizes our doctrine as "necessity without reason" and faults us for failing to explain why nature appears purposive even if it isn't really, is the misreading of a successor who could not abandon the framework of final causation. We did not fail to give purposive explanations; we explained why no purposive explanations are needed.
David Sedley has rightly argued in Creationism and Its Critics in Antiquity (2007) that we are the most thoroughgoing ancient anti-creationists: the position is directed precisely against creationist accounts (Plato's Timaeus would be the canonical foil), and the Christian-Aristotelian inheritance that built on creationism has misunderstood the atomist alternative for two millennia.34
Explanations of specific kinds of events and of particular events were governed by the principle that there are no chance events, but no attempt was made to offer explanations of the fundamental cosmic processes themselves.The Atomists, Leucippus and Democritus, p. 179
Moreover, the attribution of all events to a central feature of the mechan tic Democritean world-view, is itself attested in the fragment of Leucippus. We ought, then, to look for an interpretation of the fragment which allows it to be consistent Democritus' denial of final causation. Such an interpretation is available without forcing the texts.The Atomists, Leucippus and Democritus, pp. 176–177
The process of atomic interaction has neither beginning nor end, and any particular stage of that process is causally necessitated by a preceding stage. But exactly how the atomists saw the process as operating is obscure. This obscurity is largely attributable to the fragmentary nature of the evidence which we possess, but it may be that the statement of the theory itself was not altogether free from obscurity.The Atomists, Leucippus and Democritus, pp. 175–176
A passage from Epicurus' On Nature (208), which one might hope to be our most authoritative source, is similarly ambiguous.The Atomists, Leucippus and Democritus, p. 178
The anti-teleology of atomism has been the most polemically contested position in the history of philosophy of nature. Plato's Laws X polemic was the founding hostile reception; Aristotle's Physics II provided the more philosophically sophisticated charge that atomism reduces to "necessity without reason" and cannot account for natural regularity. The Christian-Aristotelian tradition that inherited this framing made atomism the paradigm of impious materialism for nearly two millennia. The Epicurean development of atomism in the late fourth-third century BCE modified the doctrine (atomic swerve, clinamen) to allow for free will but maintained the anti-teleological core. Modern reception: David Furley's Two Studies (1967) and Greek Cosmologists Vol. I (1987) gave the rigorous-mechanism reading its modern philosophical articulation; David Sedley's Creationism and Its Critics (2007) reframed the question as anti-creationism specifically. Sylvia Berryman (Mechanical Hypothesis, 2009) has worked on the longer history of mechanism in ancient natural philosophy. The Aristotelian charge persists in some popularizing accounts but is not the modern scholarly consensus.