- to be so, it doesn't follow that it is so.
What we can ask is whether it can make sense to doubt it.
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.
"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.
Whether a proposition can turn out false after all depends on what I make count as determinants for that proposition.
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 to be revealed.
My life shows 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.
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.]
Now do I, in the course of my life, make sure I know that here is a hand - my own hand, that is?
I know that a sick man is lying here? Nonsense! I am sitting at his bedside, I am looking