That which is there to be spoken of or thought of must be. Fragment B6

With this idea as his guiding principle, Parmenides effected a revolution in Presocratic thought. He maintained that one could only coherently speak about or think about being, rather than non-being, and using just this premise he deduced a picture of the nature of reality that completely contradicts everything experience tells us: he argued that reality is (1) eternal, (2) unchanging, (3) just one single thing, of a single, undifferentiated, qualitative type, and (4) unmoving. All later Greek philosophers, including Plato and Aristotle, had to contend with these arguments in putting forth their own theories of the nature of the world.

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