Probably the most important of the Presocratics. In his poem On Nature a goddess instructs him that reality must necessarily be, or must necessarily not be, or must both be and not be, which is impossible. Given the first option, it can be deduced that what is real must be ungenerated, imperishable, indivisible, perfect, and motionless. This, the Parmenidean One, obviously contrasts with the relative and specious appearances of things, which arise only through the opposition of two equally unreal forms, Light and Dark. So an unbridgeable gap opens between reality and appearance. Parmenides’s legacy included a profound consciousness of the conflict between reason and experience, and the potentially illusory nature of the latter: if, as Whitehead said, western philosophy is a series of footnotes to Plato, it might be added that Plato is often a series of comments on Parmenides. Certainly the contrast between the changing perceptible world and the unchanging and eternal intelligible world has exercised philosophy ever since. Zeno’s arguments against the reality of motion are usually interpreted as part of a defence of Parmenides’ system, although Zeno himself probably remained unpersuaded either by monism or pluralism.