I've been thinking about BIV (brain-in-a-vat) skepticism arguments, in part because I've read a few things on it recently. Essentially the argument goes:
1. I don't know that I'm not a brain in a vat.
2. If I don't know that I'm not a brain in a vat, I don't know [insert some item of common knowledge, e.g., I am in Toronto, or I have two hands, here].
Therefore I don't know [item of common knowledge].
I've made no secret before that I find such arguments utterly unimpressive (see here for the post on Sparacio and here for the post on DeRose), because (1) is clearly false and (2) is also false, although less obviously so. Since the point of this weblog is to help me work out and clarify various issues, I thought I would try to set up the reasons why a bit more clearly.
I think of my response to BIV skepticism as a Berkeleyan approach, because it closely resembles the way Berkeley shuts down material substance skepticism. Suppose the item of common knowledge is "I have two hands." And I do; if you ever meet me in real life, I can wave them around for you. But what would I say to BIV skepticism about my hands?
First, I do know that I am not a BIV. What would it mean for me to say, "I am a brain in a vat, not this person with this perspective, this body, etc."? I don't think it would mean anything. I am this thinking person with this perspective, this body, etc. It's true that it's entirely possible that a necessary condition for there being this thinking person with this body is that there be a brain in a vat hooked up to a machine somewhere. It's an odd thought, but it's possible; only a causal investigation would find it out. But this is a different issue from saying, "I am a brain in a vat rather than this embodied person." This statement is a well-formed sentence, but it is simply false. I am, at present, this embodied person; saying I am a brain in a vat only sounds plausible if we think (falsely) it is plausible to say we are our brains, simpliciter. But this is not what we are talking about when we are talking about ourselves.
Second, even setting aside the issue of whether I know that I am not a BIV, i.e., even if we hold this in doubt for a moment, it doesn't follow from my not knowing I'm not a BIV that I don't know that I have hands. I do know that I have hands, even if I might be a BIV, because here they are, typing on this keyboard. This will be true of any item of common knowledge about the external world. BIV skepticism, like the material substance skepticism Berkeley resoundingly refuted, depends on making an illegitimate distinction between the external world (i.e., the world we actually experience) and the external world (the other external world, whatever that would be) and holding that the latter is the real external world. We have no basis for either the distinction or the privileging of the "other" external world over what any sane person means by the external world.
Consider the following scenario. Someone comes up to me and says, "How do you know you are not a complex scintillation in the horn of an invisible pink unicorn?" I know I am not; I can tell by looking around me. But suppose I haven't thought about it, or havent thought about it clearly, and so am willing to say I don't know I'm not a complex scintillation in the horn of an invisible pink unicorn. The crazy guy continues, "If you don't know that you are not a complex scintillation in the horn of an invisible pink unicorn, you don't know that that you have two hands." But of course I know I have two hands, even if I don't know that I'm not a complex scintillation in the horn of an invisible pink unicorn: here my two hands are. The crazy guy doesn't let up. "It seems that you have two hands; but really, for all you know, it might just be a scintillation-pattern in an invisible pink unicorn's horn." But, while there are seemings that are imperfect, and so also seem to be not real (e.g., hallucinations, dreams), the seeming that I have two hands is not one of those. It is the sort of seeming that is exactly what we call 'real'. Since the crazy guy is using 'real' in an odd way, I can't agree with him unless he tells me what he means. But it doesn't matter, anyway. In the real sense of the word 'real', I really have two hands because I seem to have them. This is just what it is to "really have two hands". Any skepticism the scintillation supposition engenders is regarding the other external world, i.e., the one the crazy guy keeps calling 'real'.
There is nothing that makes the brain-in-a-vat supposition any different from the supposition that I am a complex scintillation in the horn of an invisible pink unicorn. They both are obviously false; and even if we didn't know that, they are irrelevant to skepticism about items of common knowledge. They are also crazy; but that's just from my lips to your ears, very hush-hush.