Thinking can take only one form ("it-is"), because thinking of this kind and that- which-is are inseparable (and so thinking can never be found ’’with" anything else, nor in any other form than "it-is"), This is because (there cannot be anything else, "with" which thinking might be found, since) that-which-is is unique, being necessarily whole and unmoved. The argument moves from thinking to that-which-is. It asks why thinking can take only one form and answers that the necessity of being, which makes that-which—is unique, does not permit an alternative. It is evident that Parmenides finds in being a limitation upon thought and cannot therefore have held any view that reduced being to thought. On the other hand, it is no more likely that he regarded thought as determined by being. We may grant that it is possible that the mind is somehow limited by an objective being, but Parmenides goes further than that: he says that thinking must take the form "it-is", and that that-which-is-not can be neither thought nοr spoken. How could the mind be compelled, by a necessity not its own, not merely to take a certain form, but to use a certain word? How could an objective being compel the mind to say "it-is" and to refrain from saying "it-is-not"? Parmenides’ words simply do not give a clear statement of either of these theories. It seems prudent, therefore, to seek another account of his thought, preferably one that is less sophisticated than either of these, and so more appropriate to the early fifth century B.C. It is plain that Parmenides conceives of a necessary relation between the thought "it-is" and the real world. The question is: what is that relation? The answer is to be found, I believe, in the second line of our passage.There, in the course of saying that thinking and that-which-is are inseparable, Parmenides adds a most significant, though subordinate, clause. He speaks of "that-which-is in which it is expressed".
Woodbury, Leonard, "Parmenides on Names (B 8.34-41)" (1957). The Society for Ancient Greek Philosophy Newsletter. 234.