Ancient Greek philosophy did not begin with Plato and Aristotle but with a series of thinkers in Ionia and southern Italy in the 6th and 5th centuries BC who attempted to explain the natural world through natural rather than mythological principles — a genuinely revolutionary intellectual move.

The earliest philosophers we know of — Thales, Anaximander, Anaximenes — proposed that all matter was ultimately composed of a single substance (water, the 'boundless,' air respectively). These proposals seem naive by modern standards, but the significant thing is not the specific answers but the kind of explanation being sought: systematic natural explanation rather than divine will. Anaximander's proposal of the 'boundless' as first principle is particularly interesting: it abstracts away from any specific substance and asks what the source of all opposites (hot/cold, wet/dry) could be.

Heraclitus and Parmenides represent opposite poles of early Greek metaphysics. Heraclitus argued that reality is fundamentally characterized by change and opposition — 'everything flows,' the river you step into is never the same — while Parmenides argued through rigorous logical analysis that change and multiplicity are impossible, and that reality is a single, unchanging whole. The Parmenidean argument forced subsequent philosophy to grapple with the problem of change.

Socrates' contribution was methodological rather than doctrinal: the Socratic elenchus (the cross-examination that exposes the contradictions in the interlocutor's beliefs) as a method of philosophical investigation, and the claim that wisdom begins with the recognition of one's own ignorance. 'I know that I know nothing' — or more precisely, the claim that he at least knew he didn't know, while most people didn't even know that — set the agenda for Platonic epistemology.