What was the central belief of Parmenides regarding existence?

Parmenides' philosophy centers around the concept of "being" as the fundamental nature of existence. He posited that what truly exists is eternal and unchanging, emphasizing that change and multiplicity are illusions. According to Parmenides, to think or express that non-being exists (i.e., that things can come into existence and cease to exist) is contradictory, as it implies that nothing can exist without being. This rationalist viewpoint underlines that the essence of reality is a singular, unchanging existence. His ideas challenge the perspectives of philosophers like Heraclitus, who argued for the constancy of change in the universe. Parmenides set a foundational stance in metaphysics that all subsequent philosophical thought had to either reconcile with or counter.