The Greek philosophers and thinkers who came before Socrates in the fifth century B.C.E. had various views about the nature of the world, about what it was made of, about what its principles were. Two extremes were Parmenides, who seems to have thought that change was illusory, and that true reality is one and unchanging, to Heraclitus who seems to have thought that change was the primary reality, and that it was driven by conflict.