Scepticism by a Thousand Cuts Martin Smith University of Glasgow Martin.Smith@glasgow.ac.uk Abstract Global sceptical arguments seek to undermine vast swathes of our putative knowledge by deploying hypotheses that posit massive deception or error. Local sceptical arguments seek to undermine just a small region of putative knowledge, using hypotheses that posit deception or error of a more mundane kind. Those epistemologists who have devised anti-sceptical strategies have tended to have global sceptical arguments firmly in their sights. I argue here that local sceptical arguments, while less dramatic, ultimately pose just as serious a challenge to our epistemic self-image and that some prominent anti-sceptical strategies are very poorly placed to meet it. Keywords Knowledge, Evidence, Global Sceptical Argument, Local Sceptical Argument 1. Local and Global Sceptical Arguments One well known genre of sceptical argument works by exploiting hypotheses that are incompatible with what we take ourselves to know, but which fit with the evidence we possess. As I look out of the window, I take myself to know that the sky is overcast, that trees are swaying in the wind, that there is a faint reflection in the glass etc. But what if I am radically mistaken about the origins of my perceptual experiences – what if they are not the result of anything external acting upon me? What if, in truth, there are no external objects and my experiences are spontaneous products of my own mind? This may not be a hypothesis that I'm inclined to take at all seriously, but could I really be said to know that it is false? What evidence do I have for thinking so? It's no good drawing attention to my present perceptual experiences – the sceptical hypothesis obliges me to completely reassess their significance. But if I cannot know that there really are external objects, then I cannot know that the sky is overcast or that trees are swaying in the wind or that there is a faint reflection in the glass. To make this reasoning more formal, consider the following two principles: NKWE If H is a possible hypothesis and S has no evidence against H then S is in no position to know ~H. Closure If P deductively entails Q and S knows P then S is in a position to know Q. The first of these principles captures the intuitive idea that, if a hypothesis represents a possible way that things could be, then I cannot know the hypothesis to be false unless I have evidence weighing against it. Call this principle NKWE (no knowledge without evidence). The second principle, 2 conventionally called a Closure principle for knowledge, captures the intuitive idea that one can always extend one's knowledge by deduction – that deduction will never take us from knowledge into ignorance. By NKWE, I am in no position to know that there are external objects. By Closure, if I know that there are trees swaying in the wind, then I would be in a position to know that there are external objects. Therefore, I don't know that there are trees swaying in the wind. This completes the sceptical argument. The hypothesis that there are no external objects is, of course, incompatible with anything I would take myself to know about my own body, about my immediate environment and about the world at large. We might think of this as a global or multi-purpose sceptical hypothesis that could be wielded against countless items of putative knowledge. To look at it in a slightly different way, the presupposition that there are external objects, and that our experiences result from their action upon our perceptual systems is embedded very deep within our ordinary practices of inquiry. Serious doubts about whether we can know this would call a great deal of putative knowledge into question. Not all sceptical hypotheses are like this however – some are far more limited in their ambition. Suppose I'm at the zoo and I see some black and white striped equine animals grazing in a nearby enclosure marked 'Zebras' (Dretske, 1970). In this case, I would ordinarily take myself to know that the animals before me are zebras. Now consider the hypothesis that the animals before me are, in fact, mules cleverly disguised by the zoo authorities to look like zebras. This, too, is a kind of sceptical hypothesis – it is incompatible with the proposition that the animals are zebras, but seems to fit with my evidence for thinking that they are. It's no good arguing against this hypothesis by pointing out that the animals are black and white, striped and equine and grazing in an enclosure marked 'Zebras' – for this is presumably what we would expect if the animals were mules disguised by the zoo authorities. I could gather evidence against this hypothesis by checking the animals more thoroughly – using paint stripper or conducting a DNA test perhaps – but, until such time as I've completed these further tests, NKWE would seem to land us with the conclusion that I am in no position to know that the animals are not cleverly disguised mules. By Closure, if I know that the animals are zebras, then I would be in a position to know that they are not cleverly disguised mules. Therefore, I don't know that the animals are zebras. We are confronted with an isomorphic sceptical argument – albeit one in which the stakes seem somewhat lower. If I were to take seriously the possibility that the animals are disguised mules, then all that would be called into question, perhaps, is whether I can know that the animals are zebras just by visually inspecting them and by reading the information on their enclosure. We might describe this as a local or single-use sceptical hypothesis. The presupposition that the animals are not cleverly disguised mules is one that lies relatively close to the surface – while doubts about whether we can know this might deprive us of some putative knowledge, the effects would be relatively quarantined. In thinking about how to respond to sceptical arguments, epistemologists have, to some extent, been captivated by the global sceptical hypotheses that threaten to undermine much of our putative knowledge in one fell swoop. That is, many of the anti-sceptical strategies that epistemologists have devised seem tailored, in one way or another, to global sceptical hypotheses and are much more of an ill fit with sceptical hypotheses that are merely local. One well-known anti-sceptical strategy seeks to bolster the presuppositions of our practices of inquiry with some kind of philosophical argument. One recent version of this strategy, inspired by Putnam (1981, chap. 1), appeals to externalist constraints on content. If there really were no external 3 objects, so the thought goes, then I would be completely unable to even think thoughts about trees or wind etc. for I will have had no causal contact with such things. Since I can think thoughts about trees and wind etc. I do have evidence against the sceptical hypothesis that there are no external objects, just as NKWE demands (for some discussion see Warfield, 1998, DeRose, 2000). While this particular argument may be a relative newcomer on the philosophical scene, the general idea that philosophers might be capable, in one way or another, of proving the existence of external objects is one that has a long history. In the Critique of Pure Reason, Kant famously proclaimed that the failure to prove the existence of external objects was nothing less than a scandal to philosophy (Kant, 1787, Bx1). Another much discussed anti-sceptical strategy originates in some of Wittgenstein's remarks in On Certainty. Wittgenstein conceded to the sceptic that our practices of inquiry rest upon presuppositions, such as the existence of external objects, and that we may lack any genuine evidence for thinking that these presuppositions are true. Rather than calling these presuppositions into doubt, however, Wittgenstein claimed that they are, on the contrary, certain for us. For Wittgenstein, though, this is not the kind of certainty that derives from a philosophical proof. Rather, what makes these propositions certain is the fact that they are a part of our very 'form of life' – part of the 'scaffolding' that makes inquiry possible (see Wittgenstein, 1969, §86-88, 94-95, 358-359). On one interpretation, what Wittgenstein intends here is that the presuppositions underlying our practices of inquiry are not fact-stating or truth evaluable – while they might appear to be propositions describing the world, their role is in fact entirely different (see Wright, 1985, section III, McGinn, 1989, chaps. 7 and 8, Moyal-Sharrock, 2004, chap. 2, 4, Stroll, 2005, Phillips, 2005). On this theory, to accept that there are external objects is not like accepting that there is milk in the fridge or that there's a yellow convertible parked outside – it's not to take some view as to what the world is like but, rather, to embrace a certain way of inquiring. By the same token, the sceptical hypothesis that there are no external objects doesn't describe some possible way that the world could be – and, thus, doesn't trigger the kind of evidential requirement on knowledge demanded by NKWE. Whatever else we might think of these anti-sceptical strategies, they seem to be most attractive when applied to presuppositions embedded deeply within our practices of inquiry and the global sceptical hypotheses that are incompatible with them. But the strategies seem peculiarly illequipped to deal with local sceptical arguments. We would hardly be tempted to say that the proposition that the animals are not cleverly disguised mules can be given a philosophical proof or that it is certain for us, or a part of our 'form of life'. Still less would we want to insist that this proposition is not fact-stating or truth evaluable or that the sceptical hypothesis that the animals are mules disguised by the zoo authorities doesn't describe a possible way that the world could be. As I noted above, the question of whether the animals are cleverly disguised mules is one that I could investigate in earnest. This is a straightforward empirical question of fact that could be resolved in any number of ways – paint stripper, DNA test etc. When it comes to the question of whether there are external objects, it is much more difficult to imagine how any empirical investigation could proceed – and this might make us more receptive to the thought that this can be resolved by philosophical reflection or that there is no matter of fact here. More generally, many antisceptical strategies exploit the mystique that tends to surround global sceptical hypotheses, and our associated readiness to think that they warrant some kind of special, exceptional treatment. Any such strategy will, however, be very difficult to sell when it comes to local sceptical hypotheses. 4 Perhaps, though, this isn't such an important shortcoming. As I noted above, local sceptical arguments don't appear to pose much of a threat to our putative knowledge – perhaps we could simply live with the consequences of these arguments. Furthermore, in a series of recent works, Duncan Pritchard has argued that local sceptical arguments are in fact seriously flawed (Pritchard, 2010, 2012, part 2, 2013, section 10.4). For Pritchard, we only need to deploy subtle anti-sceptical strategies when it comes to global sceptical arguments – local sceptical arguments can be answered in a much more straightforward way. In the remainder of this paper, I will argue that local sceptical arguments cannot be brushed aside – they are every bit as compelling as global sceptical arguments and we cannot simply live with their consequences. 2. Where the Threat Lies Suppose, once again, that I'm visiting the zoo, spy some black and white, striped equine animals grazing in an enclosure labelled 'Zebras' and take myself to know, on this basis, that they are zebras. As discussed, the appearance of the animals and the label on their enclosure doesn't seem to provide evidence against the sceptical hypothesis that the animals are mules cleverly disguised by the zoo authorities. But this is not to say that I have no evidence whatsoever against this hypothesis. As Pritchard points out, if I'm a normal zoo visitor, then I could be expected to have a certain amount of background knowledge about zoos – knowledge to the effect that zoos are generally reliable and trustworthy and that it wouldn't usually be in their interests to deceive the public. This background knowledge would seem to tell against the hypothesis that the animals before me are cleverly disguised mules – and might even put me in a position to know that they're not. According to Pritchard, we can answer the local sceptical argument while retaining NKWE and Closure and without resorting to any sophisticated anti-sceptical manoeuvres – we need only take note of the background knowledge at my disposal. In contrast, when it comes to the hypothesis that there are no external objects, there is no background knowledge that I can feasibly draw upon to reassure myself of its falsity – the hypothesis arguably calls all of my background knowledge into question. It is for this reason that a more sophisticated anti-sceptical manoeuvre is required in order to engage with a hypothesis like this. I agree with Pritchard that the disguised mule possibility, as standardly described, may not be effective as a local sceptical hypothesis – it's unclear whether it truly fits with my evidence, once the totality of that evidence is taken into account. But, from the fact that one proposed local sceptical hypothesis is not fit for purpose, it hardly follows that all local sceptical arguments are flawed – perhaps we simply need to choose our hypothesis more carefully. Consider the following, slightly embellished, hypothesis: Although zoos are generally trustworthy and reliable and it wouldn't usually be in their interests to deceive the public, the zoo that I'm visiting is an exception to these rules and the animals before me are mules cleverly disguised by the zoo authorities to look like zebras. Do I have any evidence against this embellished hypothesis? Clearly, it's no good pointing out that zoos are generally trustworthy and reliable and it wouldn't usually be in their interests to deceive the public – the hypothesis acknowledges as much! Citing a part of a hypothesis can hardly give me reason to reject that hypothesis. But what else could I possibly appeal to? Even if there is some further background knowledge that I can bring to bear, it too could be absorbed into the hypothesis and thereby neutralised. That is to say, the hypothesis could be further embellished to accommodate 5 any other evidence that I see fit to bring forward. The local sceptical argument can, at this point, proceed as before: If I have no evidence against this embellished hypothesis then, by NKWE, I am in no position to know that it is false. And if I am in no position to know that this hypothesis is false then, by Closure, I don't know that the animals are zebras. One wants to say, of course, that the embellished sceptical hypothesis is just a priori unlikely or implausible. There may be something to this thought. But, by NKWE, the fact that a hypothesis is a priori unlikely is not enough for one to know that it is false – for that we need evidence and, when it comes to the embellished sceptical hypothesis, we have none. Relatedly, one might complain that the embellished sceptical hypothesis is unacceptably gerrymandered or ad hoc and, thus, inferior to the commonsense zebra hypothesis that accommodates my evidence equally well. Once again, I am sympathetic to this idea. But, inferior or not, by Closure I have to be in a position to know that the embellished sceptical hypothesis is false in order to know that the zebra hypothesis is true. And, by NKWE, I need evidence against the embellished sceptical hypothesis in order to be in a position to know that it's false. These points can be made more formally. Consider the following principle: ENEA If a hypothesis H deductively entails a proposition P then P is not evidence against H. ENEA (entailments are not evidence against) has a very intuitive motivation: If H entails P, then the falsity of P would entail the falsity of H. In this case, to discover that P is true and is not false is, in effect, to eliminate one of the ways in which H could be false. Far from giving us some reason to reject H, such a discovery should, if anything, increase our confidence in it. Furthermore, it's natural to think that P could only provide evidence against H if the evidential probability of H given P were lower than the prior evidential probability of H – that is, if Pr(H | P) < Pr(H). But, if Pr is a classical probability function, we can prove that, if Pr(P) > 0 and H entails P then Pr(H | P) ≥ Pr(H) from which ENEA will follow. Let B comprise all of my relevant background knowledge about zoos and the like, whatever it be thought to include. Let Z be the proposition that the animals before me are zebras and CDM be the hypothesis that they are cleverly disguised mules. B may provide some evidence against CDM, rendering it ineffective for use in a local sceptical argument targeting my putative knowledge of Z. Provided, though, that B provides only defeasible evidence against CDM, B ∧ CDM will itself be a possible hypothesis. By ENEA, B is not evidence against B ∧ CDM. Since B, by stipulation, comprises all of my relevant background evidence, I have no evidence against B ∧ CDM, in which case, by NKWE, I am in no position to know that it is false. Since Z entails ~CDM, it also entails ~(B ∧ CDM). If I'm no position to know ~(B ∧ CDM) then, by Closure, I'm in no position to know Z. As I stated at the outset, an effective sceptical hypothesis needs to have two features – it must be incompatible with things that we take ourselves to know and it must fit with our evidence. What the foregoing reasoning demonstrates is that there is no necessary connection between the extent to which a hypothesis is incompatible with our putative knowledge and the extent to which it fits with our evidence – these are effectively independent. We can formulate hypotheses that are a perfect fit with our evidence but affect only a very targeted part of our putative knowledge – in fact, the above argument provides a general recipe for doing so. Let P be any proposition that I take myself to know 6 and let E be the totality of my evidence. Provided E doesn't make P absolutely certain, E ∧ ~P will be a possible hypothesis that is incompatible with P while being a perfect fit with E. This shows precisely why we can't just take local sceptical arguments 'on the chin'. If I concede that I cannot know that the animals are zebras, based on their appearance and the label on their enclosure, then this may not, in and of itself, have much impact – but it will set a precedent by which almost anything that I take myself to know could be undermined. A local sceptical argument can be devised to target any piece of putative knowledge that is based on evidence that is less than conclusive. We can imagine a sceptic who, rather than trying to demolish our practices of inquiry by targeting a presupposition that is buried deeply within, patiently dismantles these practices piece by piece, by targeting one superficial presupposition after another, and leaving the deep presuppositions untouched. This sceptic administers a thousand cuts, rather than a single fatal blow. Nevertheless, all or almost all of our putative knowledge is ultimately called into question. One might try to prove that global sceptical hypotheses are false or express qualms about whether they represent genuine possibilities – but none of this will engage the present sceptic. He makes no play with global sceptical hypotheses. If the reasoning in this paper is along the right track, then certain popular anti-sceptical strategies turn out to be misdirected – any viable strategy must be capable of dealing with both global and local sceptical arguments. Here is not the place to speculate on how the present sceptical arguments might best be dealt with. Suffice it to say, though, the answer lies not in scrutinising sceptical hypotheses per se – this is something of a red herring – but in examining the general assumptions about knowledge and evidence that make NKWE, Closure and ENEA seem appealing. Whether or not it is scandalous for the existence of external objects to go unproved by philosophers, the significance of such a proof, vis à vis the problem of scepticism, would be very limited. In particular, such a proof would not place our practices of inquiry on an absolutely secure footing. While the existence of external objects may be one presupposition of these practices, there are many more which philosophers simply have no business trying to prove one way or the other. What philosophers can offer here is something rather different – a way of living with these presuppositions, without falling into an epistemic crisis of conscience. References DeRose, K. (2000) 'Can we know that we're not brains in vats?' Southern Journal of Philosophy 38: 121-148 Dretske, F. (1970) 'Epistemic operators' Journal of Philosophy 67: 1007-10023 Kant, I. (1787) Critique of Pure Reason: Second Edition, Guyer, P. trans. Cambridge: Cambridge University Press, 1998. McGinn, M. (1989) Sense and Certainty: A Dissolution of Scepticism. Oxford: Blackwell Moyal-Sharrock, D. (2004) Understanding Wittgenstein's On Certainty. London: Palgrave MacMillan Phillips, D. (2005) 'The case of the missing propositions' in eds. Moyal-Sharrock, D. and Brenner, W. Readings of Wittgenstein's On Certainty. London: Palgrave MacMillan 7 Pritchard, D. (2010) 'Relevant alternatives, perceptual knowledge and discrimination' Noûs 44(2): 245-268 Pritchard, D. (2012) Epistemological Disjunctivism. Oxford: Oxford University Press Pritchard, D. (2014) 'Entitlement and the groundlessness of our believing' in eds. Dodd, D. and Zardini, E. Scepticism and Perceptual Justification. Oxford: Oxford University Press Putnam, H. (1981) Reason, Truth and History. Cambridge: Cambridge University Press Stroll, A. (2005) 'Why On Certainty matters' in eds. Moyal-Sharrock, D. and Brenner, W. Readings of Wittgenstein's On Certainty. London: Palgrave MacMillan Warfield, T. (1998) 'A priori knowledge of the world: Knowing the world by knowing our own minds' Philosophical Studies 92: 127-147 Wittgenstein, L. (1969) On Certainty. eds. Anscombe, G. and von Wright, G. eds., Trans. Anscombe, G. and Paul, D. Oxford: Blackwell Wright, C. (1985) 'Facts and certainty' Proceedings of the British Academy 71: 429-