The proem is an elaborate allegorical description of Parmenides’ journey into heaven. In a chariot drawn by “immortal mares,” he is conducted upward by the daughters of the Sun, who bring him to the “gates of the ways of Night and Day,” the keys to which “severely chastising Justice” holds. At the entreaties of the maidens, Justice opens the portals, revealing “the goddess” who addresses Parmenides:O youth, who come to our mansion in the company of immortal charioteers, welcome! It was no evil fate but right and justice that set you to travel on this way, far indeed from the path trodden by men. Meet it is that you should inquire into all things, the unshaking heart of well-rounded truth as well as the opinions of mortals in which is no true confidence at all. Yet none the less you shall learn all things, even how seeming things—all passing through each—must really be.
The goddess warns Parmenides against relying on the senses for knowledge of reality: “Keep your thought away from this way of inquiry, and by no means let much-tried custom force you this way, to ply the unseeing eye and the ringing ear and the tongue” (considered to be the organ of taste, not of speech). “Rather, judge by reason the much-disputed proof which I expound.” The much-disputed proof is strictly a priori, depending altogether on the law of identity:Well then, I shall tell you—and do you attend and listen to my word—what are the only ways of inquiry there are to think of. The first, that IT IS, and that it is impossible for it not to be, in the way of conviction, for it follows truth. The other, that IT IS NOT, and that it must needs not be,—that, I tell you, is a path that none can learn of at all. For you could not perceive what is not—that is impossible—nor even think of it; for it is the same thing that can be thought and that can be.
The goddess’s expressions are puzzling because it does not follow the rules of language (Greek or English) to say simply, “It is.” We want to know what the “it” stands for; and if “it” is (for example) a radish, the expression “A radish is” still makes no sense. A radish is what? Nevertheless, the sense of the passage is unmistakable: If there is something real (and there is), then whatever characteristics it has, it has just those characteristics, and none other. A is A. It is impossible to think of A not being A, for to say that A is not A would be in effect to say that the thing having the characteristic C does not have the characteristic C; and this would amount to saying something and immediately retracting it, so that altogether nothing would be said. It is in this sense that “it is the same thing that can be thought and that can be.” The goddess did not mean, of course, that there must be mermaids in the ocean because we can think of them. She meant that reality and thought must both be noncontradictory.
All this is perhaps innocent enough, but the goddess is going to use the principle thus laid down as a weapon to destroy belief in the reality of the world revealed by the senses. The first step is to draw the corollary “For this shall never be proved, that what-is-not is,” for “it is not possible for what is nothing to be.” The thought is that the word “nothing” means “that which is not”; consequently any sentence having the word “nothing” as its subject, and “is” as its verb, must be contradictory, “not to be thought of.”
Having established this, it is easy for the goddess to prove that “what-is is uncreated and indestructible.”...