“Like all philosophers who came after him, Plato constructed his philosophy in response to other philosophies. Plato built not only upon the thought of Socrates but also upon the theories of earlier philosophers, the so-called pre-Socratic philosophers.
The earliest philosophers of the Greek world–those of the sixth century B.C.–had wrestled with the problem of explaining physical nature by asking, what is the only basic material out of which the world is made? They all agreed that the many different kinds of things we see in the world are all transformations, changes of only one kind of thing. But they disagreed as to what is the one fundamental material out of which the many are formed: Is it water, or air, or fire?
In this way a second major philosophic problem took shape–the problem of change, of the transformations of the one into the many. Does the one change into many? Then how is it one? Are the many merely variations, transformations, or changes of the permanent, unchanging one? Then how are they many?
The two sides of this debate are represented by two major pre-Socratic philosophers: Heraclitus and Parmenides. Heraclitus, who flourished at about 500 B.C., was a solitary, pessimistic member of the nobility of the city of Ephesus. Heraclitus argued that the fundamental charcter of reality is change itself. Everything in reality, said Heraclitus, is in process, in flux, is changing. “One cannot step twice into the same river, ” he wrote, since it is endlessly flowing, changing, moving with fresh waters.
By claiming that everything is change or flux. Heraclitus denied that anything can stay the same, be identical with what is has been or will be. Thus he denied any permanence or immutability in the world. He denied the unchanging one and affirmed the changing many.
In the fiercest opposition to Heraclitus stood Permenides of Elea, in southern Italy, who flourished at about 465B.C. and wrote his philosophy in the form of poetry. Parmenides argued that not change but permanence is the fundamental character of reality. Reality is one, single, permanent,and unchanging. How can a thing change into something else? How can it be and not be? Whatever is, must be what it is, identical with itself, unchanging. Permenides therefore claimed that reality is a single, unchanging one, and he branded change as an illusion. He denied that anything can change, he denied any process of development in the world. He henied to many and affirmed the one. Change is mere appearance to the senses, Parmenides is saying; whereas truth is unchanging and is known by reason.”
(Lavine, From Socrates to Sartre)