Parmenides, an ancient Greek philosopher, proposed a radical theory of reality, famously asserting that "what is, is, and what is not, is not." According to Parmenides, existence is eternal and unchanging, and true reality is a single, undivided, and immutable entity. He argued that the concept of "nothingness" is logically impossible because it entails non-being, which cannot exist. In Parmenides' view, the idea of "nothingness" is merely a product of human perception and illusion, while true reality is characterized by unity and permanence.