Parmenides
Parmenides of Elea (ca. 520 BC - ca. 450 BC) was Greek Presocratic philosopher. He was the founder of the Eleatic school (Some scholars think that his teacher Xenophanes of Colophon (born about 570 BCE), the father of pantheism, was the real founder). Plato acknowledged that he had magnificent depths; Plato also named one of his dialogs with his name. Parmenidean idea of the nature of reality would become the basis of Plato's thinking. [1] Through Plato, Parmenides' thoughts influenced all western philosophy.
Some of Parmenides's arguments are set out in his poem entitled "On Nature", where "thought and knowledge encounter themselves head on for the first time". A fragment of this poem says: "That which is not, is not. "What-is-not" does not exist. Since anything that comes into being must arise out of what-is-not, objects, states of affairs and so on cannot come into being. Likewise, they cannot pass away, because in order to do so they would have to enter the realm of what-is-not. Since it does not exist, what-is-not cannot be the womb of generation, or the tomb of that which perishes. The no-longer and the not-yet are variants of what-is-not, and so the past and future do not exist either. Change, then, is impossible. Equally, multiplicity is unreal. The empty space necessary to separate one object from another would be another example of what-is-not. And since things cannot be anything to a greater or lesser degree—this would require what-is to be mixed with the diluting effect of what-is-not—the universe must be homogeneous".
Zeno of Elea (ca. 490 BC - ?) was a follower of Parmenides. For Theophrastus, Parmenides had a "twofold truth"; the standpoint of truth and the standpoint of popular opinion.