Skepticism (January 2004):
What can we know? Can we know anything? Can we know anything with certainty? Can we be certain of any empirical knowledge?
These are questions that a skeptic may ask. First, what is skepticism? Skepticism is the view that we lack knowledge. One can be skeptical about only certain kinds of knowledge or one can be skeptical about all knowledge. I'm concerned with Cartesian skepticism -- the claim that no empirical proposition about anything other than one's own mind is sufficiently warranted because there are always grounds for doubt and uncertainty.
Does knowledge and/or justification require certainty? If not, then the skeptical hypothesis just stated isn't a threat. (This is view which is held by Wittgenstein, the pragmatists, and many others.) I'm assuming, though, that knowledge requires certainty. Or rather, the question I'm interested in considering is whether there are any certain empirical propositions. Call this knowledge or KNOWLEDGE or anything else you like; I think it's still worth asking the question.
So what is certainty? Not surprisingly, philosophers don't agree on what it means for a belief to be certain. Russell suggests that a belief is certain if and only if there is no logical possibility that it is false. (This seems to quickly lead to skepticism.) Wittgenstein proposes that a belief is certain if and only if it does not need any justification and it can be appealed to as justification for other beliefs. (This doesn't seem to tell us much about what certainty is.) For Chisholm, a belief is certain if and only if there is no proposition more warranted than it. (This doesn't seem quite strong enough to represent certainty.) Cartesian certainty is tied up with the idea of doubt; a belief is certain if and only if it is objectively and subjectively indubitable. That is, a belief is certain just in case it is warranted and there are no true defeaters and the belief that there are no true defeaters is warranted.
The question "Of what can we be certain?" is an old one. Modern epistemology begins (roughly) with Descartes, who called many kinds of beliefs into doubt: sensory beliefs (on the grounds of illusions and hallucinations -- we sometimes seem to see things which aren't real), beliefs of an external world (on the grounds of dreams -- when dreaming we might belief we are awake), and beliefs based on reason (because we may be deceived by an evil demon in such a way as to not know we were being deceived). But famously, Descartes could not doubt the existence of his mental self.
I'm going to consider the evil demon argument a bit more. Could there really be an evil demon deceiving us? That is, it is possible for things (at the very least the external world) to appear to us exactly as they are but actually be different? If you find the idea of an evil demon hard to swallow, consider Putnam's more contemporary brain in a vat scenario: scientists remove your brain, put it in a vat, and hook it up to a sophisticated computer that feeds your brain with the same signals it would normally receive about the external world. (Yes, this is the premise of The Matrix.) If such a thing were possible, how would you know? How do we know that we are not brains in vats?
Many philosophers have responded to Descartes and skepticism. Locke's response was to concede that there is no possibility of real knowledge (beyond intution and demonstration), while holding that skepticism is no threat because we are rational to accept many things as certain in everyday life. Berkeley's answer was that whatever is perceived is real. Hume accepted the skeptical hypothesis, maintaining that we can know nothing beyond impressions and ideas and that our causal knowledge is based on custom and is uncertain. Reid, like Locke, felt that the commonsense view was not threated by philosophical skepticism, but rather than accepting skepticism, Reid believed skepticism could be defeated on the grounds that it clashes with our commonsense views. Much later, Moore held a similar commonsensist view.
I'm not sure I have any good answers to the skeptic's challenge, but there are a lot of good questions that I'll try to discuss and maybe even clarify. Are we (or is it possible that we could be) brains in vats? Could being a brain in a vat really be qualitatively indiscernible from being in the "real world"? If so, why should we prefer to know that the external world is real? If not, what would be different? Would the difference be knowable?
The more I think about these questions, the more I realize I'm a skeptic (not that this is really a surprise). I suspect a good number of people when asked "How do you know your not a brain in a vat?" think "Because I'm not" or "I'd know if I was" or "Don't be ridiculous." My first thought is "Wow, I guess I don't know. Or at least, I can't think of any empirical way to determine if I am or not." So I'm probably a skeptic about knowledge of the external world. But I think I'm a mostly harmless skeptic. My position is similar to Locke's position (at least as I presented it). I don't know if I'm a brain in a vat, but I don't think I have to know whether I'm in a brain in a vat or not to go about my life assuming that I'm not a brain in a vat (abbreviated BIV).
In other words, I don't lose sleep (except for when I'm in philosophy mode) worrying about whether I can know I'm not a BIV. It's just not something it usually occurs to me to wonder. I don't think I'm a BIV. And I seem to have some justification for thinking I'm not a BIV (even if I might not have enough justification for certainty and knowledge). I don't have any particularly justification for thinking that I am a BIV, and it just makes more sense that I'm not. Now which sounds more likely, simpler, and like a better fit with what we experience: that we are constantly being deceived or that things really are (roughly) what they seem to be? The latter. Just because I'm not positive of something (the external world), doesn't mean I feel the need to reject my belief in it entirely.
Here's another approach (based on Pascal's argument for belief in the existence of God). There are two possibilities for the way the world is: Either the external world is real and corresponds to my impressions of it, or else I'm being deceived. There are two possibilities for my beliefs about the way the world is: Either I believe the external world of my perception is real, or I believe I'm being deceived. Combining these, there are four possibilities. I have the most to gain by believing that the world is real and as I perceive it. For one thing, it's a belief that keeps me from going crazy or constantly doubting. For another, even if the world isn't real, the illusion seems quite real and it might be fun to play along (much as we know that the pieces in a chess game aren't real, but we can still enjoy chess).
A more compelling reason is that if being a BIV is really qualitatively indiscernible from having real experiences, then it is entirely appropriate to treat my sensations as being real. Taking liberty with Locke's fire example, if things are as they seem to be, and I put my hand in the fire, it will hurt. If I'm a BIV and I perform the action which could best be described as putting my hand in the fire, I will feel the sensation of pain. It will hurt. Presumably, I want to avoid the pain, so in either case I'd be wise to keep my hand out of the (real or imaginary) fire.
Or consider this scenario. Your friend tells you he doesn't want to drop acid before going to the zoo because he doesn't want to experience the sensation of being stampeded by pink elephants. (I'm assuming that zoos have yellow pigs and that to your friend while on acid yellow pigs look like pink elephants and seem to stampede.) You tell your friend not to worry; there aren't any pink elephants at the zoo, so he need not fear actually being trampled. This doesn't reassure him. He's not worried about an actual stampede with actual pink elephants; he's worried about the sensation of the stampede, a sensation he can experience without there being any elephants.
That example may seem silly, but how different is it from our typical desire to avoid being stampeded by normal elephants? My motivation for saying I don't want to be stampeded for elephants is that I associate the stampede with a sensation (a rather large amount of pain) that I am keen to avoid. I can conceive of experiencing that sensation without there being any real elephants. (After all, it just involves a certain stimulation of my brain; if that seems unlikely, consider the phantom limb sensation reported by some amputees.) If I can have the same unpleasant experience with or without the elephants, I don't want to have it either way. I don't want to spend hours sitting in Boston traffic not because I want to avoid the traffic, but because I want to avoid the annoyance I feel when sitting in Boston traffic. In other words, if being a BIV is really exactly like not being a BIV as far as my sensations are concerned, I don't think I have any reason to care whether I'm a BIV or not, and so I don't need to know if I'm a BIV or not.