Greek philosophy returned for some time to the concept of the One in the teachings of Parmenides... His most important contribution... was, perhaps, that he introduced a purely logical argument into metaphysics. "One cannot know what is not—that is impossible—nor utter it; for it is the same thing that can be thought and that can be." Therefore, only the One is, and there is no becoming or passing away. Parmenides denied the existence of empty space for logical reasons. Since all change requires empty space... he dismissed change as an illusion.