Parmenides is the founder of what is known as the Eleatic School of Greek philosophy. The Eleatics were a group of philosophers who believed that there was only one unchanging reality, and that all other things were merely an illusion.
Actually, Parmenides believed that this single, unchanging reality was the only thing that existed. He also believed that this unchanging reality was eternal and perfect.
In addition, Parmenides’ belief in the existence of an unchanging, eternal and perfect reality is known as monism. Monism is the belief that only one thing exists, and that everything else is an illusion or an appearance of some sort.
The followers of Parmenides believed that there was no change or motion in the universe at all – everything was just one thing at any given time. Even though they did not believe in change or motion, they did believe in two different types of being: Being and Not-Being.
Being was seen as a single, unchanging whole – it had no parts or divisions within it; it simply existed as itself forever and ever without changing or moving at all. Not-Being, on the other hand, had no real existence at all – it did not exist because it could not exist.
