Parmenides of Elea (5th century BC) was an ancient Greek philosopher from Southern Italy. He is one of the most signifigant of the presocratic philosophers, teaching that our experience is a multiplicity of realities and that 'all is one.'
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 miderections 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.
He and his pupils formed the so-called Eleatic School (founded by Xenophanes) and included Zeno of Elea and Melissus.
In Plato's dialogue Parmenides the eleatic philosopher and Socrates argue about dialectic.