Parmenides of Elea (ca. 520 BC - ca. 450 BC) was a Greek Presocratic philosopher. He was the founder of the Eleatic school (Some scholars think that his teacher Xenophanes of Colophon (born about 570 BC), 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. 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.
"Dialog" by Plato.
While Socrates was speaking, Pythodorus thought that Parmenides and Zeno were not altogether pleased at the successive steps of the argument; but still they gave the closest attention and often looked at one another, and smiled as if in admiration of him. When he had finished, Parmenides expressed their feelings in the following words:
Socrates, he said, I admire the bent of your mind towards philosophy; tell me now, was this your own distinction between ideas in themselves and the things which partake of them? and do you think that there is an idea of likeness apart from the likeness which we possess, and of the one and many, and of the other things which Zeno mentioned?
I think that there are such ideas, said Socrates.
Parmenides proceeded: And would you also make absolute ideas of the just and the beautiful and the good, and of all that class?
Yes, he said, I should.
And would you make an idea of man apart from us and from all other human creatures, or of fire and water?
I am often undecided, Parmenides, as to whether I ought to include them or not.
And would you feel equally undecided, Socrates, about things of which the mention may provoke a smile?-I mean such things as hair, mud, dirt, or anything else which is vile and paltry; would you suppose that each of these has an idea distinct from the actual objects with which we come into contact, or not?
Certainly not, said Socrates; visible things like these are such as they appear to us, and I am afraid that there would be an absurdity in assuming any idea of them, although I sometimes get disturbed, and begin to think that there is nothing without an idea; but then again, when I have taken up this position, I run away, because I am afraid that I may fall into a bottomless pit of nonsense, and perish; and so I return to the ideas of which I was just now speaking, and occupy myself with them.
Yes, Socrates, said Parmenides; that is because you are still young; the time will come, if I am not mistaken, when philosophy will have a firmer grasp of you, and then you will not despise even the meanest things; at your age, you are too much disposed to regard opinions of men. But I should like to know whether you mean that there are certain ideas of which all other things partake, and from which they derive their names; that similars, for example, become similar, because they partake of similarity; and great things become great, because they partake of greatness; and that just and beautiful things become just and beautiful, because they partake of justice and beauty?
Yes, certainly, said Socrates that is my meaning. cf: Parmenides, Dialogues by Plato.