An Analysis of "On Certainty" "What does this mean: the truth of a proposition is certain (OC 193)?"
"With the word 'certain' we express complete conviction, the total absence of doubt, and thereby we seek to convince other people. That is subjective certainty.
"But when is something objectively certain? When a mistake is not possible. But what kind of possibility is that? Mustn't mistake be logically excluded (OC 194)?"
Wittgenstein uses certain in two different senses in OC, subjectively certain, as an expression of a conviction or feeling about a belief, and objectively certain, which is another way of expressing a piece of knowledge as JTB. So, certainty and knowledge both have their subjective and objective counterparts, and they are often confused.
OC 194 brings up a very interesting point, i.e., the tendency is to say, "I know...," and here one thinks one is using know in the epistemological sense (as Moore's does), as a guarantee that one knows, as if a mistake is not possible. However, if a claim to know is in itself knowledge, it would seem that knowing would lose its force, it would be akin to a conviction, opinion, or intuition. Knowledge must stand up to the doubt, "How is it that you know?" Note that with a mere belief, one might respond to the question "Why do you believe that?" with the answer "I just do," and that's acceptable as a mere belief; but a claim to knowledge as JTB requires more, it requires that the belief be justified and true. And of course, Wittgenstein in challenging Moore's use by asking what would count as a justification for "I know this is a hand." Wittgenstein is telling us that Moore's use of "I know..." is akin to an expression of a conviction, not objective knowledge as Moore thinks it is. Of course, this brings us full circle, viz., that the propositions that Moore retails as knowledge hold a special place within our world picture. Moore doesn't know, in his context, what he thinks he knows.
There is a lot to unpack here, and I've just scratched the surface.