Parmenides was a pre-Socratic Greek philosopher from the city of Elea in southern Italy, active around the 5th century BCE. He is best known for his poem "On Nature," which presents his ideas about existence and reality. Parmenides argued that change and plurality are illusions, asserting that "what is" is unchanging and eternal. His philosophy laid the groundwork for later thinkers, including Plato and Aristotle. Parmenides emphasized the importance of reason over sensory experience, claiming that true knowledge comes from rational thought. His ideas significantly influenced the development of metaphysics and the study of being in Western philosophy.