Parmenides

Parmenides was a citizen of Elea, a Greek colony in southern Italy, and is said to have given the city its laws. He wrote a philosophical poem in hexameters that opens with the poet carried in a chariot by the daughters of the Sun to a goddess who reveals to him the 'Way of Truth' and the 'Way of Opinion'.

The argument of the Way of Truth is stark. What is, is; what is not, is not; what is not cannot be thought or spoken. Therefore there can be no coming-to-be (which would require passage from not-being to being), no perishing, no motion or change, and no plurality (which would require gaps of not-being between beings). Being is one continuous, complete whole like a sphere. The changing world of the senses belongs to the Way of Opinion, the error of mortals.

The conclusion contradicts all experience, but Parmenides' importance is that he was the first to reach a conclusion about reality by pure reasoning and to demand that experience yield to it. Every later Greek philosopher had to answer him: the atomists with atoms and void, Plato with Forms, Aristotle with potentiality and actuality.