Parmenides entirely identified “what can be” and “what can be thought”:

Come now, I will tell thee—and do thou hearken to my saying and carry it away— the only two ways of search that can be thought of. The first, namely, that It is, and that it is impossible for it not to be, is the way of belief, for truth is its companion. The other, namely, that It is not, and that it must needs not be,— that, I tell thee, is a path that none can learn of at all. For thou canst not know what is not—that is impossible— nor utter it; . . . . . . for it is the same thing that can be thought and that can be.

As I pointed out here, the error here comes from an excessive identification of the way a thing is known and the way a thing is. But he does this only in a certain respect. We evidently think that some things are not other things, and that there are many things. So it would be easy enough to argue, “It is the same thing that can be thought and that can be. But we can think that one thing is not another, and that there are many things. So one thing can fail to be another, and there can be many things.” And this argument would be valid, and pretty reasonable for that matter. But Parmenides does not draw this conclusion and does not accept this argument. So his claim that what can be thought and what can be are the same must be taken in a more particular sense.

His position seems to be that “to be” has one and only one real meaning, in such a way that there is only one way for a thing to be. Either it is, or it isn’t. If it is, it is in the only way a thing can be; and if it is not, it is not in the only way a thing can be. But this means that if it is not, it is not at all, in any way, since there is only one way. And in this case it is not “something” which is not, but nothing. Thus, given this premise, that there is only one way to be, Parmenides’s position would be logical.

In reality, in contrast, there is more than one way to be. Since there is more than one way to be, there can be many things, where one thing is in one way, and another  thing is in another way.

Even granting that there is more than one way to be, Parmenides would object at this point. Suppose there is a first being, existing in a first way, and a second being, existing in a second way. Then the first being does not exist in the second way, and the second being does not exist in the first way. So if we say that “two beings exist,” how do they exist? The two do not exist in the first way, but only the first one does. Nor do the two exist in the second way, but only the second one does. And thus, even if Parmenides grants for the sake of argument that there is more than one way to be, he can still argue that this leads to something impossible.

But this happens only because Parmenides has not sufficiently granted the premise that there is more than one way to be. As I pointed out in the discussion of being and unity, when two things exist, the two are a pair, which is being in some way, and therefore also one in some way; thus the two are “a pair” and not “two pairs.” So the first being is in one way, and the second being is in a second way, but the two exist in still a third way.

The existence of whole and part results from this, along with still more ways of being. “The two” are in a certain respect the first, and in a certain respect the second, since otherwise they would not be the two.

Thus we could summarize the error of Parmenides as the position that being is, and can be thought and said, in only one way, while the truth is that being is, and can be thought and said, in many ways.