Parmenides was a pre-Socratic philosopher from the Greek colony of Elea in southern Italy, active in the early 5th century BCE. He is considered the founder of the Eleatic school of philosophy. According to ancient sources, he was of noble birth and may have written laws for his city. The details of his education are uncertain, with some accounts linking him to other thinkers like Xenophanes, but modern scholars see him as an original and pivotal figure.

He is known for a single major work, a philosophical poem titled On Nature, written in the epic meter of Homer. Only fragments of this poem survive, preserved through quotations by later ancient writers. In this work, Parmenides introduced a rigorous method of logical argument focused on the nature of reality, or "what is." He argued that true reality—Being—must be one, unchanging, indivisible, and eternal. According to his view, the everyday world of change, plurality, and non-being is an illusion.

Parmenides's radical ideas established foundational problems for metaphysics and epistemology. His monism posed a direct challenge to all later theories of the physical world. He profoundly influenced subsequent Greek philosophy, including the works of Plato and Aristotle, and the Eleatic tradition he founded remained a major intellectual force for generations.