Continuing from part one, we quickly complete our treatment of G.E. Moore’s "Proof of the External World" (1939) and move on to consider "Certainty" (1941).
In the latter paper, Moore considers various "obvious" statements about his current situation (in my case now: I am writing this, I am sitting down, I have fingers, I am in my house on earth, etc.). These are all contingent truths, not truths of logic (not a priori). However, once I'm in this situation and know them to be true now, does it make sense to say (Moore asks) if they right now could be false, despite my knowledge of their truth? This is not asking the counter-factual claim of could I have decided not to be sitting here writing (of course I could have), but about the way we use the word "possible": Is it possible given that you know you are in such-and-such a location (you are certain of this) that this belief might possibly be false? Moore says no.
Another way of putting this: given that I am sitting down, can I know that I am sitting down? Is there some possibility of its actually being false right now (e.g. we're in the matrix) such that I'm not warranted in saying that my sitting position is knowledge held with the highest level of certainty (if that's not a redundant way of phrasing it)?
Moore lays out the distinctions between "I am certain.," "I know for certain," and "it is certain." Part of the point is to distinguish between certainty and infallibility. Being "certain" is compatible with actually being wrong, as in the case where the person is certain of his hand, yet is actually in the Matrix and so no claims about observation or visible bodies as spoken by the Matrix-housed individuals will turn out to be true. Moore claims that it's more likely that the theoretical considerations that give rise to such doubts are in error than that the everyday observation is faulty. He also seems to claim that it may not even be coherent to think that, for example, we're always dreaming (and so seemingly veridical perception may be faulty), because the only way we can understand dreaming to exist is by its contrast with waking perception. Do you buy any of this?
A secondary source some of us found helpful was Avrum Stroll's Moore and Wittgenstein on Certainty. For more of Moore's actually positive views and detailed arguments against other epistemologists, see his Some Main Problems of Philosophy, which can also be found online.