Parmenides first assumed that reality, or what does not change, is One thing only. Given this, anything that is not that one thing is not real. Because something that is not real cannot have an effect on what is real, nothing can divide the One. The One, by definition, cannot move or change. Since the One is the only thing that is real, what we perceive as moving and changing is not real.

Parmenides’ student Zeno of Elea (c 490–c. 430 B.C.E.) defended the idea that reality is One and immobile and unchanging by showing how positing its movement and change results in absurdities. He is famous for his paradoxes. Mellisus of Samos (fl. 440 B.C.E.) added that the One is unbounded, or in our terms, infinite, and insisted that there could not be empty space.