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Written over the last 18 months of his life and inspired by his interest in G. E. Moore's defence of common sense, this much discussed volume collects Wittgenstein's reflections on knowledge and certainty, on what it is to know a proposition for sure.
1. If you do know that here is one hand,1 we'll grant you all the rest.
When one says that such and such a proposicion can't be proved, of course that does not mean that it can't be derived from other propositions; any proposition can be derived from other ones. But they may be no more certain than it is itself. (On this a curious remark by H. Newman.)
2. From its seeming to me—or to everyone—to be so, it doesn't follow that it is so.
What we can ask is whether it can make sense to doubt it.
3. If e.g. someone says "I don't know if there's a hand here" he might be told "Look closer".—This possibility of satisfying oneself is part of the language-game. Is one of its essential features.
4. "I know that I am a human being." In order to see how unclear the sense of this proposition is, consider its negation. At most it might be taken to mean "I know I have the organs of a human". (E.g. a brain which, after all, no one has ever yet seen.) But what about such a proposition as "I know I have a brain"? Can I doubt it? Grounds for doubt are lacking! Everything speaks in its favour, nothing against it. Nevertheless it is imaginable that my skull should turn out empty when it was operated on.
5. Whether a proposition can turn out false after all depends on what I make count as determinants for that proposition.
6. Now, can one enumerate what one knows (like Moore)? Straight off like that, I believe not.—For otherwise the expression "I know" gets misused. And through this misuse a queer and extremely important mental state seems tobe revealed.
7. My life shews that I know or am certain that there is a chair over there, or a door, and so on.—I tell a friend e.g. "Take that chair over there", "Shut the door", etc. etc.
8. The difference between the concept of 'knowing' and the concept of 'being certain' isn't of any great importance at all, except where "I know" is meant to mean: I can't be wrong. In a law-court, for example, "I am certain" could replace "I know" in every piece of testimony. We might even imagine its being forbidden to say "I know" there. [A passage in Wilhelm Meister, where "You know" or "You knew" is used in the sense "You were certain", the facts being different from what he knew.]
9. Now do I, in the course of my life, make sure I know that here is a hand—my own hand, that is?
10. I know that a sick man is lying here? Nonsense! I am sitting at his bedside, I am looking attentively into his face.-So I don't know, then, that there is a sick man lying here? Neither the question nor the assertion makes sense. Any more than the assertion "I am here", which I might yet use at any moment, if suitable occasion presented itself.—Then is "2 x 2 = 4" nonsense in the same way, and not a proposition of arithmetic, apart from particular occasions? " 2 x 2 = 4" is a true proposition of arithmetic-not "on particular occasions" nor "always" but the spoken or written sentence "2 x 2. = 4" in Chinese might have a different meaning or be out and out nonsense, and from this is seen that it is only in use that the proposition has its sense. And "I know that there's a sick man lying here", used in an unsuitable situation, seems not to be nonsense but rather seems matter-of-course, only because one can fairly easily imagine a situation to fit it, and one thinks that the words "I know that . . . " are always in place where there is no doubt, and hence even where the expression of doubt would be unintelligible.
11. We just do not see how very specialized the use of "I know" is.
12. —For "I know" seems to describe a state of affairs which guarantees what is known, guarantees it as a fact. One always forgets the expression "I thought I knew".
13. For it is not as though the proposition "It is so" could be inferred from someone else's utterance: "I know it is so". Nor from the utterance together with its not being a lie.—But can't I infer "It is so" from my own utterance "I know etc."? Yes; and also "There is a hand there" follows from the proposition "He knows that there's a hand there". But from his utterance "I know . . . " it does not follow that he does know it.
14. That he does know takes some shewing.
15. It needs to be shewn that no mistake was possible. Giving the assurance "I know" doesn't suffice. For it is after all only an assurance that I can't be making a mistake, and it needs to be objectively established that I am not making a mistake about that.
16. "If I know something, then I also know that I know it , etc." amounts to: "I know that" means "I am incapable of being wrong about that". But whether I am so needs to be established objectively.
17. Suppose now I say "I'm incapable of being wrong about this: that is a book" while I point to an object. What would a mistake here be like? And have I any clear idea of it?
18. "I know" often means: I have the proper grounds for my statement. So if the other person is acquainted with the language game, he would admit that I know. The other, if he is acquainted with the language-game, must be able to imagine how, one may know something of the kind.