91. If Moore says he knows the earth existed etc., most of us will grant him that it has existed all the time, and also believe him when he says he is convinced of it. But has he also got the right ground for his conviction? For if not, then after all he doesn't know (Russell).
92. However, we can ask: May someone have telling grounds for believing that the earth has existed for a short time, say since his own birth?--Suppose he had always been told that,--would he have any good reason to doubt it? Men have believed that they could make rain; why should not a king be brought up in the belief that the world began with him? And if Moore and this king were to meet and discuss, could Moore really prove his belief to be the right one? I do not say that Moore could not convert the king to his view, but it would be a conversion of a special kind; the king would be brought to look at the world in a different way.
Remember that one is sometimes convinced of the correctness of a view by its simplicity or symmetry, i.e., these are what induce one to go over to this point of view. One then simply says something like: "That's how it must be."
93. The propositions presenting what Moore 'knows' are all of such a kind that it is difficult to imagine why anyone should believe the contrary. E.g. the proposition that Moore has spent his whole life in close proximity to the earth.--Once more I can speak of myself here instead of speaking of Moore. What could induce me to believe the opposite? Either a memory, or having been told.--Everything that I have seen or heard give me the conviction that no man has ever been far from the earth. Nothing in my picture of the world speaks in favor of the opposite.
94. But I did not get my picture of the world by satisfying myself of its correctness; nor do I have it because I am satisfied of its correctness. No: it is the inherited background against which I distinguish between true and false.
95. The propositions describing this world-picture might be part of a kind of mythology. And their role is like that of rules of a game; and the game can be learned purely practically, without learning any explicit rules.
96. It might be imagined that some propositions, of the form of empirical propositions, were hardened and functioned as channels for such empirical propositions as were not hardened but fluid; and that this relation altered with time, in that fluid propositions hardened, and hard ones became fluid.
97. The mythology may change back to a state of flux, the river-bed of thoughts may shift. But I distinguish between the movement of the waters on the river-bed and the shift of the bed itself; though there is not a sharp division of the one from the other.
98. But if someone were to say "So logic too is an empirical science" he would be wrong. Yet this is right: the same proposition may get treated at one time as something to test by experience, at another as a rule of testing.
99. And the bank of that river consists partly of hard rock, subject to no alteration or only to an imperceptible one, partly of sand, which now in one place now in another gets washed away, or deposited.
100. The truths which Moore says he knows, are such as, roughly speaking, all of us know, if he knows them.
Ludwig Wittgenstein On Certainty