Early Greek philosophers pondered about the meaning of existence, about the origin of life, and the derivation of other elements the world consisted of. These natural philosophers all shared the belief that in order for changes to occur in nature (for example, rain falling from the sky) there had to be "something" that all things came from and returned to. Instead of continuously relying on ancient Greek myths, philosophers attempted to come up with logical explanations and answers to their questions of where "things" came from and how they "transformed" in nature. Two of these early philosophers were Parmenides and Heraclitus. Both gave contributions from their thoughts on natural philosophy, however, their assumptions were completely opposite from one another. Heraclitus believed that everything in the world and in nature is constantly changing, yet Parmenides argued that nothing changes for the simple fact that everything that exists, can only exist as is because it came from nothing and therefore cannot transform into something else. .
"You cannot step into the same river twice, for fresh waters are ever flowing in upon you." This statement sums up what Heraclitus" philosophy about the nature of being was. He looked at everything as being in a constant state of flux, or continuous change, and so reality to him was "a chain of transitions." Heraclitus explained that "nothing what is now will be the same tomorrow," and with that it is said that he planted the idea of impermanence (not being permanent or continual) into Greek thought, and after Heraclitus Greek philosophy was not the same anymore. Another thing that Heraclitus believed was that fire is the origin from which everything else arises, and said that fire itself is the symbol of perpetual change because it transforms a substance without being a substance itself. Heraclitus believed in a universal reason which guides everything that happens in nature and humans, he called this reason God, or logos.
