This is an interesting paper by Micah Sparacio at ISCID on external world skepticism. While Sparacio's response is a good one, I am puzzled by a few things.
First, in what sense do we really have 'intuitions' about the brain-in-a-vat argument for skepticism? As far as we genuinely have any intuitions, i.e., common-sense assumptions and inferences, at all, they seem for the simple rejection of the brain-in-a-vat scenario. We are not brains in vats. Do you need proof? Look in a mirror and see. This is fairly close to Moore's principle. Sparacio claims that Moore's argument begs the question, but he does not show that it does, and I see no reason to think it does. Moore's argument in essence makes the point that our starting-point for talk about the external world is what we ordinarily take to be the external world. Not only does this not beg the question, it is difficult to see on what basis one could reject. This point was made long ago by Berkeley: brain-in-a-vat scenarios, like the sort of material substrate against which Berkeley argued, are based on confusion in what they are explaining. If what you are explaining when you are talking about the external world is the world you actually see and hear and touch, you cannot go behind it and talk about something else (e.g., brains in vats) without changing the subject - you are no longer talking about the external world as such but something else, which (at best) has a relation to the external world. Seen in this way, it seems to me that Moore, the contextualists, and Sparacio are all approximating, but perhaps to a degree waffling around, the basic point: it is pointless to talk about what follows 'given the skeptical scenario'; what we are given is what we actually call 'the external world'. In effect, this skeptical scenario, like others, exhibits an ignoratio elenchi. If this is so, however, Sparacio's conclusion that his proposal only weakens external world skepticism's bite is excessively modest.
Second, we can only talk about what follows from the brain-in-a-vat scenario anyway if we can take it that it is genuinely consistent for me to say that I am a brain in a vat. But this has been called into question (Putnam's arguments on the subject tend in this direction), so it cannot be taken for granted. Further, we have good reason to be suspicious of the whole thing, because the so-called 'intuitions' that undergird the scenario seem to be nothing but the most tenuous sort of analogy: we can think about brains in vats, we know that brains are (somehow) linked with thought, so we can make a story in which we really are nothing but brains in vats. But of course this requires that 1) immaterialism about mind be false; 2) we are not, as we often do in stories, creating the intellectual equivalent of an optical illusion by playing games with reference and sense; 3) the final stage, that we are nothing but brains in vats, is not contradicted by evidence; 4) this actually is relevant to our knowledge of the external world. All four of these assumptions are false. We have no reason to think the brain-in-a-vat scenario a genuine possibility; what is more, we have reason to think it is not; what is more, it is irrelevant to the actual issue of the external world.
In effect, the brain-in-a-vat skeptical scenario can no more deal with Berkeley's anti-skeptical arguments than abstracted-material-substrate skeptical scenarios could. Berkeley is still right in his basic critique, even if one finds problems with his positive proposal; the problem has been solved since the 18th century. It is still interesting to look at other ways in which such scenarios are tripped up (which is why the discussion is still interesting), but Sparacio's claim that external world skepticism has 'intuitive force' is arbitrary and implausible.
