Two men, William [W] and Thomas [T], are speaking to each other:
W – I tell you, things can't possibly be as you say. Just listen and I will explain.
T – But you cannot explain; for the two us to come to any agreement there would have to be common ground between us and if things are as you say, then that is not possible. And yet the fact we are having this conversation proves that it is.
W – Well now who's talking nonsense? What on earth do you mean?
T – First, let us get our facts straight. You've said that we cannot know the reality of things outside ourselves, correct?
W – No, that is not what I said.
T – Of course it is! You said that you can't be sure that an oak tree in Oxford is in any way similar to one in Italy.
W – Yes, I did say that.
T – If you can't know that they are similar in any way then how is it that you can refer to both of them as oak trees or as trees at all? If we cannot know there is anything similar about them then we can't know enough to even refer to them with the same word 'tree'. With this way of thinking all of reality begins to break down. What if the individual thing I am referring to with the word 'tree' is actually a cat and what you are referring to with the same word 'tree' is actually a butter knife? If this were the case then our conversation right now would be pure nonsense. And yet this is obviously not true because you do agree that our conversation right now is intelligible, right?
W – No, I don't.
T – ?!?
And thus William proves for us that Nominalists are schizophrenic; using words which truly represent something in reality while denying the existence of that very reality.