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The external word scepticism is the view that the evidence we have for the truth of our beliefs about the world falls short of what is required for knowledge. The most powerful sceptical argument proceeds by means of radical sceptical hypotheses. A sceptical hypothesis involves a scenario in which we are systematically deceived about the world but everything in our experience is exactly as it would be if we were not deceived. Take, for example, your belief that you have hands. Clearly, this belief is incompatible with the claim that you are a brain in vat (BIV). That is, if you are a BIV, then it is false that you have hands. So using the BIV hypothesis, the skeptic argues as follows. (1) You don’t know that the BIV hypothesis is false. (2) But if you don’t know that the BIV hypothesis is false, you don’t know that you have hands. Therefore, you don’t know that you have hands. The BIV argument is clearly valid and has two plausible premises. Yet its conclusion amounts to saying that we have no knowledge of the external world. / The first premise seems quite plausible because you are simply unable to know that you are not a BIV. If your experience could be exactly as it is and yet you are a victim of a sceptical scenario, then on what basis you ever hope to say that you have a genuine experience, but not an illusory one? That is, your evidence or experiences about the world cannot really help you to tell whether or not you are not a BIV. The second premise also seems plausible, because it is supported by the closure principle. This principle tells us that if you both know an entailment and its antecedent, then you also know the entailed proposition. But since you don’t know that you are not a BIV, you don’t know that you have hands either. / If these premises are true, then the conclusion cannot be false. Two intuitively plausible premises can thus be employed to generate a valid argument which produces this rather devastating conclusion. So, in this sense, the BIV argument constitutes a paradox. But what is one to do about it? There are various responses to the sceptical argument that have been addressed in contemporary epistemology. There are at least four strategies to rebut the sceptical argument: (1) the Moorean response, (2) the fallibilist response, (3) relevant alternatives and denial of closure response, and (4) contextualist response. This paper surveys contemporary work on scepticism. As the above grouping suggests, we will not address all the important responses, responses like semantic and epistemological externalism; but will focus on the ones that occupy center stage. After presenting the BIV sceptical argument, we will argue that all these responses to scepticism are far from unproblematic in that they fail to deal effectively with the problem posed by skeptical hypotheses. / The Moorean strategy to rebut scepticism consists of rejecting its first premise on the basis of denying the conclusion. So Moore clearly accepts the second premise stating that if we don’t know that the sceptical alternative is false, then we don’t know that we have hands. But he rejects the sceptical conclusion and uses it, together with second premise, to conclude that we know that the sceptical hypothesis is false. Although this modus tollens is as good as its rival, it does not really succeed in dealing with the sceptical threat. Because we need to explain how we know it to be false without simply deducing it from what we claim to know about the world. Without such an explanation, one cannot be said to know that the sceptical hypothesis is false. / But can we really know that the skeptical alternative is mistaken? To be sure, answering this is far more difficult than it might seem. But Steup (2006) says that fallibilism provides an answer to this question. Fallibilism rejects the Cartesian account of knowledge and suggests, in essence, that our fallible evidence gives us knowledge of the world. But, on the other hand, if the BIV argument is using Cartesian knowledge throughout, then its conclusion says nothing new. So, according to this response, the sceptical scenario cannot “disturb us” because the skeptic is just right in contending that we have no Cartesian knowledge of the external world, including the falsity of the BIV hypothesis. This simply concedes that we are unable to know that the sceptical alternative is mistaken. However, it is not easy to convince oneself that refuting the BIV argument takes no more than that. After all, the problem posed by the sceptical alternative is that we seem unable to know that it is mistaken. / A different response to the sceptical argument is given by Dretske and Nozick. They reject the second premise by denying the closure principle. Both Dretske and Nozick seek to provide a definition of what knowledge is that backs this move. Dretske (1970) thinks that if knowledge entails conclusive reasons, then closure is false. For him, closure fails because even though S knows that S’s belief that S has two hands entails that S is not a BIV, the second premise is false. That is, you can come to know that you have two hands without knowing whether or not you are not a BIV, because the alternative of your being a BIV is not relevant. This clearly blocks the sceptical conclusion by denying closure. Similarly, Nozick (1981) applies his tracking account of knowledge to the sceptical argument and claims that since knowledge implies the sensitivity requirement, closure fails. On Nozick’s account, if S knows that p, then S would not believe that p if p were false. What this means is that while your belief about your hands are sensitive, the belief that you are not a BIV is not. So you can know that you have hands without also knowing that you are not a BIV, despite the fact that you also know that the former entails the latter. After noting some unpleasant consequences of denying closure, we argue that this attempt to block the skeptical conclusion is also too concessive. While it seems to save our ordinary claims to knowledge, it simply admits that you cannot know that you are not a BIV. So the skeptical threat remains unsolved. / One final anti-skeptical view that we will consider is contextualism. This view holds that the truth of a knowledge-ascription varies with the context in which the attribution is uttered. Contextualism invokes a distinction between the high standards for knowledge that the skeptic usually demands and the lower standards that may be in place when we are making ordinary knowledge attributions. These standards can both be legitimate, because they appeal to different contexts. So the contextualist response to skepticism is roughly this: both the skeptic and the anti-skeptic, like Moore, are right when making epistemological evaluations. That is, the skeptic speaks truly when he says that you don’t know that you are not a BIV, because he is employing a high knowledge standard. On the other hand, Moore also speaks truly when he claims that you know that you are not a BIV. They do not contradict each other because, in their proper contexts, they use different standards of knowledge. Cohen (2005) thinks that this response to skepticism saves both our ordinary knowledge-claims and closure. Additionally, contextualists also claim that their position explains why we find the sceptical conclusion both compelling and preposterous. However, we also find this way of rebutting skepticism too concessive. The contextualist response to the BIV argument amounts to saying that we should agree with the skeptical conclusion. But allowing this kind of compromise with the skeptic seems to be, not really a response, but a defense of skepticism. Moreover, an appeal to the idea that knowledge is a context-sensitive notion will clearly be unhelpful because the skeptical threat does not trade on any epistemic standards at all. Thus the skeptical conclusion remains unblocked.
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