Biographical Background
Parmenides lived in the late sixth and early fifth centuries BCE and came from Elea in southern Italy. He is the central figure of the Eleatic school and one of the founders of Western ontology. If Heraclitus emphasized change, Parmenides emphasized the unchanging character of Being. The tension between becoming and being became one of the defining problems of Greek philosophy.
Parmenides wrote a philosophical poem, usually called On Nature. In it a goddess reveals two paths: the way of truth and the way of opinion. The way of truth says that what is, is, and what is not, is not. The way of opinion describes the world as mortals ordinarily experience it.
Philosophical Thought
Parmenides' central claim is that Being is and non-being is not. The statement seems simple, but its consequences are radical. Thought and speech must be about what is. Absolute non-being cannot be thought or spoken of as something. From this, Parmenides argues that true Being cannot come into existence, because it would have to come either from being, in which case it already is, or from non-being, which is impossible. Nor can it perish, because that would mean passing into non-being.
True Being must therefore be ungenerated, imperishable, whole, continuous, one, and motionless. Change and plurality appear to the senses, but strict reason shows them to be problematic. If something changes, it becomes what it was not; if there are many things, they must be separated by what is not. Both seem to invoke non-being.
Parmenides thus forced philosophy to distrust immediate sense experience and to examine the logical conditions of thought. Later philosophers had to respond to him. Atomists introduced atoms and void; Plato distinguished Forms from sensible things; Aristotle developed potentiality and actuality. All of these were attempts to explain change without falling into contradiction.
His importance is enormous because he made Being itself a philosophical problem. Metaphysics as the study of being owes much to Parmenides. He showed that philosophy must take logic seriously, and that ordinary experience cannot simply be accepted without conceptual examination.