What Is, Is
Parmenides of Elea wrote a philosophical poem in which a goddess sets out two routes of inquiry, and the whole of subsequent Greek metaphysics is a response to it.
The Way of Truth starts from something apparently harmless: what is, is; what is not, is not. You cannot think or speak of what is not, because there is nothing there to think about. Since "nothing" cannot be an object of thought, any reasoning that requires it is illegitimate.
From this, Parmenides derives properties of what-is with genuine rigour:
| Property | Because |
|---|---|
| Ungenerated | Coming to be requires starting from what-is-not |
| Imperishable | Ceasing to be requires becoming what-is-not |
| Indivisible | Division requires gaps of what-is-not between parts |
| Unchanging | Change requires becoming what one is not |
| Complete | Lacking anything means lacking some being |
Each follows from refusing to let "what is not" do any work. Motion needs empty space to move into, but empty space is what-is-not, so there is none. Plurality needs boundaries separating things, but a boundary is where one thing is not, so there are none. What exists is one, unchanging and whole.
The Way of Opinion is the second part: the world as the senses report it, which the goddess presents while marking it as unreliable. Parmenides is thus the first thinker to hold that reason and the senses systematically conflict, and to side with reason without hesitation.
The conclusion is absurd and the argument is not obviously bad, which is exactly why it mattered so much. Nobody could simply dismiss it. Every subsequent Greek philosopher had to find the flaw or accept the consequence, and their responses are the next century of philosophy: the pluralists keep unchanging elements but let them mix, the atomists bite the bullet and admit void as a genuine kind of thing, and Plato distinguishes two levels of reality so that both Parmenides and Heraclitus can be partly right.
Common pitfall: dismissing Parmenides because the conclusion is obviously false. That is not a refutation, an argument is refuted by finding the false premise or the invalid step, and nobody managed it for a very long time. The productive question is always which premise to reject, and Greek philosophy is largely a record of different answers.