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.
To understand why Parmenides and many since him have been drawn to this position, compare thought or speech to sight (this line of thinking is not only illuminating, it is also the very line of thought that often leads philosophers to make the claim in the first place): Imagine trying to see something that is not there to be seen. It is impossible. It is true that Hamlet could see a dagger even though there was no actual dagger before him, but there was a hallucination to see. Imagine if there were both no dagger and no hallucination. The he would not see anything at all.
Well, reasons Parmenides (though not, of course, with Hamlet in mind), why should speech or thought be any different? We cannot see what is not there to be seen, so why should we be able to refer to what is not there to be referred to? If something does not exist, in other words, we cannot think about it and we cannot speak about it.
Because of this profound link between thinking and being, Parmenides claims we cannot make any statements of non-being. So we cannot, for instance, speak about unicorns, even to say that they do not exist. In fact, we can never claim that anything does not exist, because anything that does not exist cannot be spoken about.
Certainly, it puts a strain on science and everyday chitchat to rule out all statements on non-being, but, actually, Parmenides wants to go much further than ruling out talk of unicorns. He is not only ruling out the path of not being, he is also ruling out the third path—the path that mixes both being and not being. The third path is the path that human beings generally travel, the path that the senses pull us down. Statements of the third path include such innocuous sounding claims as "the Sun is hot", "the sky is blue", and "cats are soft." To say that the sun is hot is also, implicitly, to say that it is not cold or lukewarm. To say that the sky is blue is to implicitly assert that it is not any other color. To say that cats are soft is to implicitly claim that they are not rough or sticky or hard. To make any claims about qualities, about changes, about almost anything at all, is to implicitly and illegally talk about non-being.
Parmenides thus drastically restricts the rational inquiry through which one can get at the nature of reality; this rational inquiry cannot make use of any premise that involves non-existence. The rational inquiry must begin with the premise "it is" and deduce the nature of reality from out of it. What Parmenides ends up deducing is that "what is" is ungenerated and unperishable, unchanging, perfect, one, and continuous.
The general form of argument he uses for each of these conclusions is along the following lines: whatever is is X, because if not X then it is not-X, and in order to explain what it is for anything to be not-X we must talk about "what is not". Since we have already seen the meaninglessness of any thought or statement involving "what is not" we can conclude that whatever is is X.
To see how this argument works in specific cases we can look at how Parmenides argues against the possibility of generation, destruction, and change. To argue against generation, Parmenides claims that there is implicit non-being in birth since it implies prior non-existence ("I will not permit you to say or to think that it grew from what is not for it is not to be said or thought that it is not" 28B8) Though Parmenides does not actually lay out a similar argument against the possibility of destruction, it is generally assumed that this is because he sees it as obvious that a parallel argument can be given—just as generation must be generation out of non-being, destruction must be destruction into non-being.
The impossibility of change follows from the impossibility of generation since characteristics and properties cannot come into being any more than objects can. To say, e.g., 'X is becoming rarified' implies that there was a time when X's rarefaction did not exist.
In addition to being eternal and unchanging, Parmenides also deduces that the Real is "perfect" and that it is one and continuous. In claiming that "what is" is perfect, he seeks to show that it has definite limits and is spherically shaped. In claiming that "what is" is one and continuous, Parmenides is probably making the strong claim that all of reality is one—that is, that the class of things that exist contains just one member (rather than only the weaker claim that "what is" is all internally alike, which he undoubtedly means to assert as well). It is difficult, though, to see how Parmenides thought he could have argued for this stronger claim. One suggestion that has been made (for instance by Kirk, Raven, and Schofield on page 251 of The Presocratic Philosophers) is that Parmenides thought he had an argument for this conclusion based on the identity of indiscernibles. Such an argument might have gone as follows: in order for X to be separate from Y there must be some Z, distinct from both, separating them. Z must either be or not be. But Z cannot not be because that is incoherent. And if, instead, it is, then there is nothing to distinguish it from either X or Y since being does not admit of degrees.
After providing us with this startling account of reality, Parmenides (or his goddess) then does something even more startling: he gives us a full-blown, Milesian-style cosmogony. In other words, after arguing that the world as we observe it does not exist, he then proceeds to give an account of the origins of the world as we observe it. This move has confounded commentators for millennia, and though there are several theories to account for this oddity, none of them are particularly satisfying.
The first possibility is that Parmenides provides the cosmogony as a parody. On this reading, the cosmogony is thoroughly condemned by Parmenides and is meant to appear as self-refuting. Though this explanation would be consistent with the rest of his thought, it leaves some troubling puzzles. First and foremost among these is the question of why Parmenides would go to the enormous trouble of providing a detailed cosmogony (his cosmogony is more detailed than most) if his only point was to ridicule the entire field of cosmogony.
The other possibility, no less troubling, is that Parmenides lightens up a little at the end of his work: that he admits that there are, in fact, two levels of reality. The first is the higher, realer level that he has just described in the "On Truth" section of his poem. The second is the inferior, lower level that corresponds with our observations. This lower level of reality would not have full being, on this view, but it would also not be an utter delusion either. Some ways of describing it would be more accurate than others. The cosmogony, then, would be the best possible account of the inferior world of appearances.
Something like this division of two worlds is what Plato presents in the Republic. According to Plato, there is a world of appearances in which human beings reside and then a more real world of forms to which human beings have intellectual access. It is possible that Parmenides anticipated Plato's division by a few decades or even that he inspired it. If he did, though, his students certainly did not follow him in this. Zeno is adamant that plurality and motion are absolutely impossible, and Melissus is just as adamant that there is only one real thing in the world. This would not, though, be the first time that the followers were more dogmatic than their leader.