(born c. 515 BC) Greek philosopher, leader of the Eleatics. His general teaching has been reconstructed from the few surviving fragments of his lengthy verse composition On Nature. He held that the multiplicity of existing things, their changing forms and motion, are but an appearance of a single eternal reality, Being. This doctrine, which was formulated as the principle that all is one, entails that all claims of change or of non-Being are illogical. Because of his method of basing claims about appearances on a logical concept of Being, he is considered a founder of metaphysics. Plato's dialogue Parmenides discusses his thought.