Of their existence or of their non-existence?

Of their existence.

Yes, Ctesippus, and we just now proved, as you may remember, that no man
could affirm a negative; for no one could affirm that which is not.

And what does that signify? said Ctesippus; you and I may contradict all
the same for that.

But can we contradict one another, said Dionysodorus, when both of us are
describing the same thing? Then we must surely be speaking the same thing?

He assented.

Or when neither of us is speaking of the same thing? For then neither of
us says a word about the thing at all?

He granted that proposition also.

But when I describe something and you describe another thing, or I say
something and you say nothing--is there any contradiction? How can he who
speaks contradict him who speaks not?

Here Ctesippus was silent; and I in my astonishment said: What do you
mean, Dionysodorus? I have often heard, and have been amazed to hear, this
thesis of yours, which is maintained and employed by the disciples of
Protagoras, and others before them, and which to me appears to be quite
wonderful, and suicidal as well as destructive, and I think that I am most
likely to hear the truth about it from you. The dictum is that there is no