Parmenides, (born c. 515 bce), Greek philosopher of Elea in southern Italy who founded Eleaticism, one of the leading pre-Socratic schools of Greek thought. His general teaching has been diligently reconstructed from the few surviving fragments of his principal work, a lengthy three-part verse composition titled On Nature.
Parmenides held that the multiplicity of existing things, their changing forms and motion, are but an appearance of a single eternal reality (“Being”), thus giving rise to the Parmenidean principle that “all is one.” From this concept of Being, he went on to say that all claims of change or of non-Being are illogical. Because he introduced the method of basing claims about appearances on a logical concept of Being, he is considered one of the founders of metaphysics. Plato’s dialogue the Parmenides deals with his thought.