After almost a century of philosophy based on the general Milesian pattern Parmenides cast the whole project into doubt by maintaining that the fundamental nature of reality has nothing to do with the world as we experience it. He went much further than Heraclitus in tempering our reliance on the senses; he rejected the senses as entirely misleading and pressed on reason alone to reveal the truth. Through his use of unadulterated reason he came to conclusions about the nature of the world that seemed to suggest not only that the theories of earlier thinkers were utterly unintelligible, but that the very questions they asked were the wrong questions to be asking. In so doing, he entirely changed the course of philosophy, demanding new attention for fundamental problems (such as the problems of change and plurality) and setting the standard for a new, more exacting level of rational argument.
Parmenides was born in 515 B.C. in the city of Elea in southern Italy. There are reports that he was a student of Xenophanes, and it seems plausible that his work was in part a reaction to Xenophanes' pessimistic epistemology. There is also some speculation that he was associated with the Pythagoreans at one time, since they, like he, were based in southern Italy. But, if this is true, then he completely rejected their influence.
Like Xenophanes, Parmenides wrote in verse. His poem "On Nature" is in Homeric hexameters and includes many Homeric images, especially from the Odyssey. With obvious reference to the poetic tradition, Parmenides begins his poem with the invocation of a divine source. Where the poets would invoke the muses in order to give themselves authority, Parmenides opens by describing a similarly fanciful scene: he is taken in a chariot to meet a goddess, who tells him that she will teach him all things about the nature of reality and assures him of the certainty of what she is about to reveal. But, she adds in a philosophical twist, he must still assess for himself all the arguments that she presents. Parmenides' use of this old poetic, mythological ruse might have been more than literary reference. Given that Parmenides was about the put forth what might well be the single most radical and counterintuitive worldview on record, it was probably not a bad idea on his part to bolster his credibility with an appeal to divine authority.
Putting all of his faith in the power of abstract reason, Parmenides argues in his poem that genuine knowledge can only involve being, and that non-being is literally unspeakable and unthinkable. Using only the premise that "what is" is and what "is not" is not, he proceeds to deduce the nature of reality. The reality he arrives at bears no resemblance at all to the world we experience around us through our senses.
According to Parmenides, the senses are entirely deceptive, and reason alone can lead us to truth. The nature of the world, then, can only be gotten at through a rational inquiry. When starting out on a rational inquiry, according to Parmenides, there are only two logically coherent possibilities: either you begin your inquiry with the premise that the subject of your inquiry exists or you begin with the premise that it does not exist.
But the second of these possibilities, according to Parmenides, is utterly meaningless. It is, therefore, not a real possibility at all. Parmenides bases this claim regarding the path of "it is not" on the assertion that, "that which is there to be thought or spoken of must be" (28b6). What he seems to be getting at here is an idea that has had extraordinary pull for philosophers through contemporary times: one cannot possibly refer to what is not there to refer to.