Heraclitus is the philosopher of the eternal change. He expresses the notion of eternal change in terms of the continuous flow of the river which always renews itself. Although it is likely that he took this thesis to be true, universal flux is too simple a phrase to identify his philosophy. Heraclitus accepted only one material source of natural substances, the fire. He held that fire is the primal element out of which everything else arises. Fire is the origin of all matter, through it things come into being and pass away. Fire itself is the symbol of perpetual change because it transforms a substance into another substance without being a substance itself. "There is something about the nature of fire that gives insight into both the appearance of stability and the fact of change"(Palmer 24). From this vision, he obtained some remarkable conclusions. Change occurs within a range of opposites. The unity of opposites means that opposites cannot exist without each other - there is no day without night, no summer without winter, no warm without cold, no good without bad. Comparing the union of opposites with the contrary tension of a bow and a lyre is perfectly in harmony with his theory of flux and fire. Also, he focused on aspects such as Logos, which ruled change in an orderly manner. This cosmic order made change a rational phenomena rather than the chaotic, random one it appeared to be. The basic principle of change to explain the universe is evident throughout Heraclitus" beliefs.
Another philosopher, Parmenides believed that Being is rational, that only what can be thought can exist. Since "nothing" cannot be thought, without thinking of it as something, there is no nothing, there is only Being. A Greek philosopher from Elea in southern Italy and founder of the Eleatic school, Parmenides was the most influential of the Presocratic philosophers. He is the first philosopher to insist on a distinction between the world of appearances and reality.
