If you are surprised that one can know something and not be able to say it, you are perhaps thinking of a case like the first. Certainly not of one like the third.
Consider this example. If one says “Moses did not exist”, this may mean various things. It may mean: the Israelites did not have a single leader when they withdrew from Egypt–Or: their leader was not called Moses
-Or: there cannot have been anyone
who accomplished all that the Bible relates of Moses-Or: etc. etc. We may say, following Russell: the name “Moses”-can be defined by means of various descriptions. For example, as “the man who led the Israelites through the wilderness” , "the man who lived at that time and place and was then called 'Moses " “the man who as a child was taken out of the Nile by Pharaoh’s daughter” and so on. And according as we assume one definition or another the proposition "Moses did not exist? acquires a different sense, and so does every other
proposition about Moses. -And if we are told “N did not exist”, we do
ask: “What do you mean? Do you want to say . . . … Or . . . … etc.?” But when I make a statement about Moses, am I always ready to substitute some one of these descriptions for “Moses”? I shall perhaps say: By “Moses” I understand the man who did what the Bible relates of Moses, or at any rate a good deal of it. But how much? Have I decided how much must be proved false for me to give up my proposition as false? Has the name “Moses” got a fixed and unequivocal use for me in all possible cases? Is it not the case that I have, so to speak, a whole series of props in readiness, and am ready to lean on one if another should be taken from under me and vice versa?
Consider another case. When I say “N is dead”, then something like the following may hold for the meaning of the name “N”: I believe that a human being has lived, whom I (1) have seen in such-and-such places, who
(2) looked like this (pictures), (3) has done such-and-such things, and
(4) bore the name “N” in social life. Asked what I understand by
“N”, I should enumerate all or some of these points, and different ones on different occasions. So my definition of “N” would perhaps be “the man of whom all this is true”. But if some point now proves false?-Shall I be prepared to declare the proposition “N is dead” false- even if it is only something which strikes me as incidental that has turned out false? But where are the bounds of the incidental?-If I had given a definition of the name in such a case, I should now be ready to alter it.
And this can be expressed like this: I use the name “N” without a fixed meaning. But that detracts as little from its usefulness, as it detracts from that of a table that it stands on four legs instead of three and so sometimes wobbles.)
Should it be said that I am using a word whose meaning I don’t know, and so am talking nonsense? Say what you choose, so long as it does not prevent you from seeing the facts. (And when you see them there is a good deal that you will not say.)
Wittgenstein, Philosophical Investigations