Scepticism and Irrationalism

1.1 Introduction

The scepticism to which I will try to provide an answer in this book is neither Academic scepticism, which said that there is but one thing one can know, namely that one can know nothing else, nor Pyrrhonian scepticism, which said that one cannot even know that, nor Cartesian scepticism, which said that even logic and mathematics are dubious, but Humean scepticism. Humean scepticism allows that each of us can have a good deal of egocentric knowledge about our own beliefs, feelings, and perceptual experiences. It also allows that logical truths can be known. But it denies that one can progress by logical reasoning from perceptual experience to any genuine knowledge of an external world, if there is one. Since its target is not all knowledge, but only knowledge of the external world, Humean scepticism is not self-undermining; it does not exclude the possibility that one can know (perhaps on logical grounds) that there can be no genuine knowledge of an external world.

Humean scepticism seems to follow from the conjunction of the following three propositions, which I will number since I will often be referring back to them in the course of this book:

(I) there are no synthetic a priori truths about the external world;

(II) any genuine knowledge we have of the external world must ultimately be derived from perceptual experience;

(III) only deductive derivations are valid.

I will also refer to these three propositions as, respectively, the anti-apriorist thesis, the experientialist thesis, and the deductivist thesis. They entail that for any factual statement h to constitute knowledge, there must exist true premises e that report perceptual experiences and from which h is logically derivable. But if h speaks about the external world and e speaks only of perceptual experiences, h goes beyond e and therefore cannot be logically derived from e.

1.11 Antisceptical Strategies

I will now attempt to list the main strategies that have been adopted in response to this simple and seemingly cogent argument. These strategies fall into two groups: those that deny one or other of the above three propositions, and those that try to cope with scepticism while retaining all three. I will begin with the former.

Apriorist strategy: deny proposition (I), modifying proposition (II) accordingly, and claim that there are certain truths about the structure of the world (for instance, that the Law of Causality reigns throughout it) that are not analytic but can be known a priori, being necessarily true. This of course was the essence of Kant's answer to Hume.

Transcendental argument strategy: modify proposition (II) and claim that there are certain principles (for instance, a Principle of Induction) that are not themselves established in the ordinary inductive way but can be proved by an argument of the form: This principle must be true if scientific knowledge is to be possible; but scientific knowledge exists; therefore this principle is true'.

Conjecturalist strategy: deny proposition (II) and claim that scientific knowledge about the external world is not derived from experience but is essentially conjectural and is only negatively controlled by experience. This of course is the essence of Popper's answer to Hume.

Nondeduetivist strategy: deny proposition (III) and claim that an inference does not have to be deductive for it to be valid.

I now turn to strategies that do not (or need not) involve the denial of any of those three propositions.

Probabilist strategy: retain proposition (III) but add that probability logic, or the logic of partial entailment, is a legitimate extension or generalisation of classical logic, and that in the case of a hypothesis h that is not entailed by evidence e we may be able to establish, with the help of probability logic, that h is more or less strongly confirmed by e. This of course is the essence of the solution of the problem of induction offered by Keynes, Jeffreys, Reichenbach, Garnap, Hintikka, and others.

Phenomenalist strategy: close the alleged gap between perceptual experience and hypotheses about the external world by claiming that physical objects just are clusters or sequences of actual or possible perceptions. Mach took this position; so did Russell in his (1914) and Carnap in his (1928).

Vindicationist strategy: vindicate the inductive method by showing, not that it will work well, but that, if any method for predicting the future will work at all, then an inductive method will work at least as well as any other. Reichenbach, Feigl, and Salmon have taken this position.

Pragmatist strategy: concede that there is no purely epistemological answer to scepticism, but add that we may nevertheless accept and reject hypotheses rationally, in the light of our utilities.

Naturalist strategy: concede that there is no epistemological answer to scepticism, but add that scepticism is of only academic interest because nature has endowed us with a cognitive machinery that is too robust and vigorous to be affected by it. Thus we can make our peace with scepticism, acknowledging it in philosophical moments and forgetting it when we hang up philosophy and go out to dine or play backgammon. This was, of course, Hume's "answer" to his own problem. Pascal had previously taken a rather similar view.

Of course, not all of these strategies are mutually exclusive. The transcendental argument and apriorist strategies may be combined, as they were by Kant; so may the probabilist and phenomenalist strategies, and so may the nondeductivist and naturalist strategies. However, I would like to insist that two strategies that are often bracketed together, namely the nondeductivist and probabilist, should be regarded as quite distinct. There are versions of probabilism that come rather close to nondeductivism; I am thinking of those subjective or personalist versions that allow us to distribute initial probabilities according to taste. But the kind of probabilism with which we will be concerned tries to provide a nonarbitrary and impersonal measure of both initial and relative probabilities. A system of probability logic that is to achieve this has to be perfectly explicit. The contrast with nondeductivism could hardly be more glaring. The latter does not try to generalise classical logic: it postulates a peculiar, nondeductive relation between statements whereby e can provide inductive proof for h without entailing it; and far from being perfectly explicit, nondeductivists maintain a sealed-lips policy concerning the nature of this mysterious relation.

There are some other defences against scepticism to which antisceptics have resorted but which hardly deserve to be called strategies and which I will deal with forthwith in a summary way. One is the claim that scepticism is an incoherent position or that "the sceptic" sets a standard for knowledge that is, in P.F. Strawson's words (1959, p. 34), 'self-contradictorily high'. Well, I can discern nothing incoherent or self-contradictory in the three propositions that I presented above as posing the problem of scepticism. And I trust that readers will discover nothing incoherent or self-contradictory in the case for scepticism that will be presented in the two chapters after this. I could have made my task easier if I had done what many antisceptics do, which is to set up a dummy called "the sceptic" and make him say various things that can be brushed aside as incoherent, self-contradictory, or whatever; but then the real case for scepticism, which is a cogent one, would have been left unanswered.

Another defence against scepticism on which I will dwell only briefly is what has been called the Paradigm Case Argument. I like to think that I knocked out this argument in (1957), though I should have done it more neatly. The argument runs–some of the descriptive expressions that figure in debates between defenders and critics of common sense have a meaning that cannot be explained purely verbally and can be taught only ostensively, by pointing to exemplars of them (Malcolm, 1942, p. 361). Let F be such an expression, and let it be a tenet of common sense that there are things that are F Let X be either a sceptic who doubts, or a speculative metaphysician who denies, that anything is F. Now X either (1) does, or (2) does not, understand the meaning of F If (2), then X does not know what he claims to be doubting or denying. So assume (1). But if X understands F then he must at some time have been directly acquainted with something that was F.

Now I believe that there is one class of descriptive expressions to which this argument does indeed apply, namely those that denote what Locke called simple ideas. I presumably could not understand such expressions as 'mauve patch' or 'musty smell' unless I had at some time seen a mauve patch or smelt a musty smell. But if this argument is to assist defenders of common sense against Humean scepticism it must extend beyond simple ideas to at least some items in the external world. For Humean scepticism does not deny that we have a considerable egocentric knowledge of mental existents. But any attempt to extend the Paradigm Case Argument to expressions referring to things that transcend our subjective experiences of them was forestalled long ago by Frege's sharp distinction between the sense and the reference of a denoting expression. As he said, 'In grasping a sense, one is not assured of a reference' (1892, p. 58). To which we may add that to know what an expression refers to does not ensure that one has grasped its sense. Consider the expression 'material thing'. Norman Malcolm claimed that this is one of those expressions 'the meanings of which must be shown and cannot be explained' (1942, p. 361). If that were so, Berkeley would, presumably, stand refuted by the fact that this expression is meaningful, since the only way in which we could have come to understand it would be by acquaintance with material things. But is Malcolm's twofold claim true? Let us postpone the question whether its meaning cannot be explained and begin by asking whether it can be shown.

Suppose that Malcolm had wanted to convey the meaning of 'material thing' to a person X who speaks a language unknown to Malcolm and who understands no English. This should not be a serious obstacle, on Malcolm's view, since the meaning of that expression has in any case to be conveyed in an ostensive and nondiscursive way. Imagine that Malcolm holds up a book, nods his head, and pronounces 'Material thing' slowly and clearly. He repeats this performance with various other objects in the room; then he takes X to the window, points to a tree, nodding and pronouncing 'Material thing', and repeats this with other outside objects. In order to disabuse X of the idea that the expression refers just to visible objects he next blindfolds X and places a heavy object in his hand, pronouncing 'Material thing' as he does so. He then removes the bandage, points to the shadow cast by an object, and shaking his head says 'Not material thing'. In due course X's eyes shine with comprehension. He now starts pointing to things to which Malcolm had not pointed, happily exclaiming 'Material thing' each time, and Malcolm nods approvingly. To Malcolm's delight, X then points to a rainbow and, shaking his head, says 'Not material thing'. X then turns his attention to a mirror. He points to it from behind and says 'Material thing'; then stepping in front of it he first points to his own eyes, ears and nose, then gestures towards the mirror and says 'Not material thing'. Malcolm interprets all this as X's way of indicating that while the mirror itself is a material thing, the image of X reflected in it is not; and Malcolm again nods approvingly. Let us suppose that on all subsequent occasions X applies this expression only to things to which Malcolm would apply it. Would all this mean that he has grasped its sense?

Quine envisaged a situation where the natives in a certain area appear to a linguist, who is trying to decode their language, to use the word gavagai to refer to what we call rabbits; but perhaps it means something like 'rabbity patch' (1960, ch. ii). Rather similarly, X may be, for all we know, an idealist of a Berkeleyan kind who understands 'material things' to mean 'sensible objects that are not intangible'; he may have entirely missed the idea of the materiality of material things.

As to the second part of Malcolm's claim, namely that the meaning of 'material thing' cannot be explained, this seems plain false. Descartes said that material things are extended (i.e. three-dimensional) and that no two of them can simultaneously occupy the same place, to which Newton added that they resist displacement (possess inertia) and Locke added that they resist compression. What is wrong with a discursive explanation along those lines? It would have been easier to convey the meaning of this expression to X if it had been possible to explain to him that a molecule that is too small to be sensible is nevertheless a material object, and that the Milky Way, though too vast and far-flung to be tangible, is also a kind of material object. I conclude that Malcolm got it the wrong way round: the sense of 'material thing' can be explained and cannot be shown; and that we can grasp its sense is no refutation of the idealist's claim that it has no reference.

1.12 Science as a Language Game

There is a defence, if that is the word, of induction that was nicely summarised by Wesley Salmon thus: 'induction needs no defence because it is indefensible' (1968, p. 24). I suspect that this view can be traced back to Wittgenstein's pronouncement: 'What has to be accepted, the given, is — so one could say — forms of life' (1953, p. 226e). A form of life, according to Wittgenstein, is informed by rules. Some of its rules will have a more or less subordinate status, and may be defended by appeal to rules higher up. But at the top of the hierarchy of rules there will be a supreme rule (or set of rules) that cannot be defended by appeal to anything higher up; nor should it be defended by appeal to anything outside the form of life, for that would subordinate it to something alien. It is indefensible. Now apply all this to that form of life called science and assume, for the sake of the argument, that its supreme rule is a principle of induction. Then this principle is indefensible just because it reigns supreme. As Ayer put it, inductive reasoning

could be irrational only if there were a standard of rationality which it failed to meet; whereas in fact it goes to set the standard: arguments are judged to be rational or irrational by reference to it....

[The sceptic's] demand for justification is such that it is necessarily true that it cannot be met. ... When it is understood that there logically could be no court of superior jurisdiction, it hardly seems troubling that inductive reasoning should be left, as it were, to act as judge in its own cause. (1956, p. 75)

To this Bartley riposted: 'The nub of the skeptical ... objection was not, after all, simply the argument that comprehensive justification is impossible. It was, rather, that since comprehensive justification is impossible, the choice between competing ultimate positions is arbitrary' (1964, p. 15). If science constitutes one form of life, then presumably magic constitutes another; and everything that Ayer said about the former's supreme principle would apply pari passu to the latter's. Wittgenstein himself touched on this difficulty in a posthumously published work.

608. Is it wrong for me to be guided in my actions by the propositions of physics? Am I to say I have no good ground for doing so? Isn't precisely this what we call a 'good ground'?

609. Supposing we met people who did not regard that as the telling reason. ... Instead of the physicist, they consult an oracle. ... Is it wrong for them to consult an oracle and be guided by it? — If we call this "wrong" aren't we using our language-game as a base from which to combat theirs? (1969, p. 80e)

Wittgenstein provided no answer to this very pertinent question. For what answer could he have given? Had he said that people are wrong to allow themselves to be guided by oracles, he would have conceded that there is at least one form of life, or language game, that can be judged and found wanting by criteria external to it. Then why should not our own language game be judged by criteria external to it? Yet how could he say that people are right to allow themselves to be guided by oracles, when he himself preferred to be guided by 'the propositions of physics'? The only solution open to him would seem to be the relativist thesis that it is right for anyone playing the language game of magic to be guided by oracles rather than by the propositions of physics, and vice versa for anyone playing the language game of Western science.