Parmenides

Parmenides set out his philosophy in a poem, of which substantial fragments survive, in which a goddess reveals to a young man the “way of truth”. Starting from the claim that what is not cannot be thought or spoken of, he argued that what is must be ungenerated, imperishable, whole and unchanging. The changing world we perceive belongs to the “way of opinion”.

How exactly to read the poem is still debated, but its arguments set the agenda for the philosophers who followed, who tried to explain change and plurality without falling foul of them. Plato named a dialogue after him. His dates are approximate: he was active in the early fifth century BCE.