Haraclitus and Parmenides are often seen as opposites, Heraclitus proposing everything is constantly changing, and Parmenides saying that change is impossible.
Heraclitus - all is in flux.
"You cannot step twice into the same river, for fresh waters are ever flowing in upon you."
"To god all things are fair and good and right, but men hold some things wrong and some right." "Nothing ever is, everything is becoming."
Parmenides - nothing changes.
"Though canst not know what is not - that is impossible - nor utter it; for it is the same thing that can be thought and that can be."
"How, then, can what is be going to be in the future? Or how could it come into being? If it came into being, it is not; nor is it if it is going to be in the future. Thus is becoming extinguished and passing away not to be heard of."
"The thing that can be thought and that for the sake of which the thought exists is the same; for you cannot find thought without something that is, as to which it is uttered."
Obviously, things seem to change. Therefore, P. seems to be saying that the world of our senses must be an illusion and not what is actually real. P. is credited as being the first to use logic to make an argument for how the world must be.
"[Parmenides] argues with devastating precision that once one has said that something is, one is debarred from saying that it was or will be, of attributing to it an origin or a dissolution in time, or any alteration or motion whatsoever. But this was just what the Milesians had done. They supposed that the world had not always existed in its present cosmic state. They derived it from one substance, which they asserted to have changed or moved in various ways—becoming hotter or colder, drier or wetter, rarer or denser—in order to produce the present world-order."(from Guthrie 1965, 15-16)
Before Parmenides, the idea existed that there was some sort of thing out of which the universe as we know it evolved out of. Anaximander called this "the Boundless," a thing within which opposites existed in a latent form before being unleashed into, our out of, the universe. It becomes complicated to know what is meant by this - is it a physical thing, is it a conceptual thing, etc. Parmenides, nonetheless, denies that such a thing could have existed out of which varying opposites came to be, for what is is, and there was never a time in which it could not have existed or been different.
P. recognizes the apparent contradictions between logic and sense experience, and sides with logic, saying the senses must be in error. Reality should be judged by reason. This is “all that can be said about what truly exists,” and reality is thus revealed as “something utterly different from the world in which each one of us supposes himself to live,” a world which is nothing but a “deceitful show” (Guthrie 1965, 51). However, P. still laid out an extensive cosmogeny, which begs the question, if the world of physical appearances isn't real, why bother?