In On Certainty, Wittgenstein returns again and again to the proposition, "I have never been on the moon." He takes this as an exmple of the sort of the thing it is not possible to doubt. §286 is a good example:
What we believe depends on what we learn. We all believe that it isn't possible to get to the moon; but there might be people who believe that that is possible and that it sometimes happens. We say: these people do not know a lot that we know. And, let them be never so sure of their belief—they are wrong and we know it.
If we compare our system of knowledge with theirs then theirs is evidently the poorer one by far.
That was written in 1950. Suppose we said the following today:
What we believe depends on what we learn. We all believe that it is possible to get to the moon; but there might be people who believe that that it is not possible and that it never happened. We say: these people do not know a lot that we know. And, let them be never so sure of their belief—they are wrong and we know it.
If we compare our system of knowledge with theirs then theirs is evidently the poorer one by far.
In fact, there are people who believe that we have never been on the moon. These are the so-called "cranks" who think the Apollo program was a hoax. As I see it, they hold two beliefs: (a) it was indeed impossible to get to the moon in 1950, (b) an "official" assurance that something happened is no reason to believe that something in fact did happen. After that, of course, there is the discussion of the "anomalies" that reveal that the evidence NASA provides is faked, and so forth.
In any case, since these people do not believe that anyone has been to them moon, they do not know that we have been there. There have a different "system of knowledge", we might say.
