Suppose you are brought up to believe that you are a brain in a vat supervised by intelligent machines. Long ago humans abandoned their frail bodies for a safer life as a detached brain (it had something to do with global warming—bodies getting too hot and so on). Everyone you talk to believes this story, and you believe it implicitly. Your experience is really dream experience, with no veridical perception at all. You firmly believe that your life is a dream conducted in a vat. And let us suppose that the story is true (though we could also suppose it to be false but you still firmly believe it): you are a dreaming brain in a vat and that is precisely what you believe. You take yourself to know all this—just as we take ourselves to know that we are not brains in vats dreaming our life away. You never suppose that what you are experiencing is real, and it never is. In fact, the question has never even occurred to you.
Now suppose you go to a philosophy class (i.e. you dream about being in a philosophy class) and the teacher introduces you to what she calls “skepticism about the dreaming world”. She invites you to consider a possibility that has never occurred to you before: that you are not a brain a vat but something else entirely—an awake, embodied, perceiving, mobile animal that is constantly confronted by what she calls an “external world”, i.e. a set of real objects that match the content of your experience. This strikes you as an implausibly exotic hypothesis, hard to take seriously; it seems decidedly hokey. The teacher then presses the following question: how can you rule out, on the basis of your experience, that you are not a “brain in a head”, i.e. the aforementioned kind of being. You cite what you have been taught from birth—the truth that you are a safely ensconced-in-a-vat dreamer. You might even try to argue that you can tell from internal features of your experience that it must be a dream: but the teacher patiently points out that what she calls “waking perception” could be just like a dream from the inside. Thus it is logically possible that you perceive an external world (as she puts it), even though you have always believed, quite truly as it happens, that you are non-stop dreamer, without even a body. The idea that you have a body strikes you as particularly bizarre, especially given the obvious problems associated with such entities (breakdowns, diseases, etc). The professor then announces her devastating conclusion: you don’t know that you are a dreamer in a vat—for all you know, you are this weird thing “a veridical perceiver”. You find this disturbing, mind-boggling even, but you are gripped by the argument; you decide to spend your life trying to refute this kind of skepticism. You set out to prove that there is no external world of the kind hypothesized.
Here is another scenario of the same type. You are brought up to believe that most of the people around you are zombies, enjoying no inner life. A disease has prevented their brains from producing minds. When you look inside their heads you see signs of brain damage, which are not present in those who never had the disease. So you believe that some people are zombies and some are not, and that you can tell the difference; moreover, suppose all this is true. You therefore take yourself to know that there are no other minds in the case of certain people—despite their behavioral similarity to those with minds. You can interact with them and even talk to them, but you know that there is nothing going on inside. This is just part of common sense so far as you are concerned. Now you go to a philosophy class and are invited to consider another possibility: that the so-called zombies have been misdiagnosed—they have minds after all. The teacher argues that you cannot logically rule out that hypothesis, since nothing in their behavior or neurological condition entails that they are zombies. Your supposed evidence for their lack of a mind is compatible with their really having a mind. So your confident belief in the absence of other minds in their case is misplaced. This, the teacher says, is called the “problem of other zombies”: how can you demonstrate that these people are zombies? Your belief to this effect may be a true belief, she points out, but it is not a justified belief, since all your evidence is compatible with an alternative hypothesis—that these people actually do have minds. You are being irrational if you deny this hypothesis by insisting dogmatically on the hypothesis you grew up believing. Again, you are gripped by the argument and resolve to spend your life trying to prove that the people who are usually regarded as zombies really are zombies.
Or suppose you are brought up to believe that the world was created five minutes ago, and in point of fact it was (something about God and his designs for us). Then you are confronted with the skeptical hypothesis that the universe is billions of years old, with many millions of years of biological evolution. You find this a bizarre and incredible story, but you are asked what proof you can give that it is false. This is called “skepticism about the short past”. Again, you find yourself stumped. Or suppose you are taught that emeralds are grue, so that they will all turn blue in the year 2050; and suppose this is a true belief, since nature is not in fact uniform. Still, your philosophy professor asks you how you know that emeralds are not really green and thus will remain green after 2050. She asks how you can prove that nature is not uniform, i.e. how you know that induction yields false beliefs about the future. The skeptical hypothesis here is that induction rests on the uniformity of nature: how can that hypothesis by excluded?
The lesson I want to draw from these suppositions is that the confidence of the aliens is no more unjustified than ours. We are epistemologically on a par: the cases are perfectly symmetrical. They have true beliefs about their situation, but the skeptic can produce alternatives that call their usual justifications into question; and so do we. It is just that we invert each other’s natural beliefs and skeptical alternatives. Just as we regard their natural belief system as strange, so they regard ours as strange: our skepticism is as alien to them as theirs is to us. Nor does it really change the case to suppose their natural beliefs are false: they will still find it hard to accept that they are not brains in vats even if that is their situation, as we find it hard to believe we are brains in vats even if we are. The truth will seem strange to both of us, given what we have been brought up to believe. What seems exotic to one set of believers will seem banal to the other set. Thus belief in the external world, other minds, the past, and the uniformity of nature will strike our alien believers as radical and outrageous, just as we think their habitual beliefs are radical and outrageous.
We can make the same point by considering our own dreams and waking experiences. Suppose the skeptic insists that what we call our dreams are really veridical perceptions and that our putative perceptions are really dreams. How can we refute this inversion? We think our waking experiences are not dreams, but the skeptic says they might be; and we think our dreams are pure fantasy, but the skeptic says they might be accurate perceptions. The alien skeptical professor suggests that the dreams of the brains in a vat might be veridical perceptions—and the same point could be made about the subset of our experiences we call dreams. It is natural to us to believe that the experiences we have at night while asleep are merely fantasies, but can we really rule out the possibility that they put us into contact with a real world? The usual skeptical dream hypothesis is that all our putative waking experience could be but a dream, but there is also the hypothesis that our putative dreams might be veridical perceptions. This flies in the face of what we naturally assume, but it is hard to see how the possibility can be refuted. The alien dreamers are us writ large—even when we really are dreaming, how can we know we are? What counts as skepticism depends on what you come into the seminar room believing.