Parmenides of Elea had stuck to the traditions – his only written work is called “On Nature”. It is divided into two parts, called “The Way of Truth” and “The Way of Opinion” respectively. Burnet: “It is clearly meant to indicate that Parmenides had been converted, that he had passed from error (night) to truth (day), and the Two Ways must represent his former error and the truth which is now revealed to him.” So, the philosophy of Parmenides is a kind of blend between the teaching of Pythagoras and previous speculations of the Ionian philosophers (Thales, Anaximander, Anaximenes, and Heraclitus).
Parmenides based his philosophy on the Pythagorean principle “It is the same thing that can be thought and that can be”. Simply speaking, if it can be thought, then it can exist; if it cannot be thought, then it does not exist.
He further asserted that truth cannot be known through our senses, which are deceptive, but only through pure reason, or Logos (which is different from Heraclitean Logos). Therefore, the movement and change which we can observe are mere illusions. His argument is as follows: Nothing does not exist, because you always think of Something. Our world, if it was created, should have come either from Nothing or from Something. It could not have been created out of Nothing, since Nothing does not exist. It could not have been created out of Something, for there is nothing, except for what is. In the same manner, the world cannot cease to be, for it would have to go into the realm of Nothing. Therefore, the past and future do not exist. Change, then, is impossible.
Further, if Something exists, it cannot exist to a greater or lesser degree. It simply either is or is not. Therefore, multiplicity is unreal, because in order for objects to be multiple they must be separated by void, and void is Nothing, and Nothing does not exist. If there is no Many, but One, then there is as much of it in one place as in another, which makes Anaximenes’ rarefaction and condensation impossible.
World also must be a sphere (influence of Pythagorean theory again), because sphere is the only figure that has no reason to be in one direction, than in another.
Bailey: “Parmenides’ world is thus a finite, eternal, indivisible, immovable, spherical, corporeal mass: motion, change, variety, birth, and death, all that we know by the experience of our senses are mere delusions.”
In his theory, Parmenides has shown that if we consider one substance as the fundamental of the whole world, we are inevitably led to the logical conclusion, which is impossible to accept. Thus, Parmenides marked the end of the Monistic theory and also set the condition for the new Pluralistic theory, represented by Empedocles. The disciples of Parmenides, Zeno and Melissus, had only supported his theory, Zeno with his paradoxes and Melissus with his additions.
Parmenides’ influence on the whole later philosophy was so great, that one philosopher once said that the whole subsequent Western philosophy was only interpretation of Parmenides.
John Burnet. Early Greek Philosophy.
Cyril Bailey. The Greek Atomists.
Bertrand Russell. The History of Western Philosophy.