6th/5th century BC
Parmenides was a Greek philosopher from Elea in southern Italy, who founded the Eleatic School of Philosophy.
Parmenides offered the view that the changing world, visible to the senses, is too perishable and unstable to be ultimate reality.
In his principle work, a lengthy two-part verse composition, he held that the multiplicity of existing things are but an appearance of a single eternal reality.
His doctrine of 'all is one' contrasts with the opposite view of his contemporary philosopher, Heraclitus, who maintained that 'all is change'.
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From the Internet Encyclopedia of Philosophy