Parmenides of Elea (5th century BC) was an ancient Greek philosopher born in Elea, a Greek city on the Southern coast of Italy. He is one of the most significant of the presocratic philosophers. He demonstrated that the common-sense belief in the reality of the physical world of plurality and change (The Way of Seeming) is mistaken; and showed contrary to common-sense, the reality of the world is 'One Being' (the Way of Truth): an unchanging, ungenerated, indestructible whole.
He was the founder of the Eleatic school, which also included Zeno of Elea and Melissus.
His work On Nature exists only in fragments and is made up of two parts as well as an introductory discourse. The Way of Truth discusses that which is real and the Way of seeming discusses that which is illusory - the misdirections and errors caused by the imperfect senses. Under the Way of Truth, he argued that the existence of a thing implied that it could not have "come into being" because "nothing comes from nothing." Moreover he argued that movement was impossible because it requires moving into "the void", and Parmenides identified "the void" with nothing, and therefore (by definition) it does not exist. That which does exist is The Parmenidean One which is timeless, uniform, and unchanging.
Under 'way of seeming', in the same work, he set out a contrasting but more conventional view of the world, thereby becoming an early exponent of the duality of appearance and reality. For him and his pupils the phenomena of movement and change are simply appearances of a static, eternal reality.
In Plato's dialogue Parmenides the eleatic philosopher and Socrates argue about dialectic.