When a sceptic asked of your proposition 'ah, but what if you're a brain in a vat on Mars?' the appropriate response in almost all cases is simply this: 'so what?'
There's an unwritten. unspoken, often unthought clause to all our theories and propositions, and that is, that when we are arguing they are true or false--good or bad, we mean as theories or propositions to describe this world we live in, whatever this world is when looked at from above it: An illusion, part of a multi-verse full of other worlds with differing truths, or whatever else it could be.
To point out that there could be a scientist, looking over your brain as it thinks within its vat, who can see this extra context about your world that you don't know (a.k.a that you're imagining it), is in many ways the same kind of claim as saying that there's a God looking at you with the extra information that he created you and put your world into motion. It's the same in the following sense: that despite the fact that it is extra information that would change your perceptive on your theories and propositions, it doesn't really affect the quality of your theories and propositions given that you live in the world that you do, a.k.a one that almost entirely ignores this "extra information".
For this reason, let Moore say to the sceptic, "damnit, I have a hand!"* And let it be unsaid that he means 'at least in my world I do'
*'cause I see it' isn't really a good reason to believe you have hands in and of itself, but that's quite another matter