1.   The Scope of Skepticism

        The scope of skeptical challenges can vary in a number of important respects, such as the following:

(1) Knowledge versus justified belief;

(2) Certain knowledge versus knowledge in general;

(3) Contingent propositions versus both contingent propositions and necessary propositions;

(4) Inferentially justified beliefs (or inferential knowledge) versus all beliefs (or all knowledge), whether inferentially justified or non-inferentially justified;

(5) Global versus local.

        In addition, a philosopher may either view skepticism as a thesis that is plausible, or - as in the case of Descartes - may view the temporary adoption of a skeptical point of view as an important first step in arriving at an account of the foundations of knowledge that will lead to a refutation of skepticism.

Comments

(1) Both the methodological skepticism of Descartes in his Meditations, and the substantive skepticism of Keith Lehrer in his essay “Why Not Skepticism?” (The Philosophical Forum, vol. 2, 1971, pp. 283-98, and reprinted in Louis Poyman (ed.), The Theory of Knowledge, Third edition (Belmont, CA: Wadsworth, 2003, pages 56-63) are directed (at least explicitly) at knowledge claims.  Thus Descartes, in his First Meditation, when referring to "ancient and commonly held opinions" - such as the opinion that there are external objects which one perceives - says "nor will I ever lose the habit of deferring to them or of placing my confidence in them, so long as I consider them as they really are, i.e., opinions in some measure doubtful, as I have just shown, and at the same time highly probable, so that there is much more reason to believe in than to deny them."  (René Descartes, Meditations, reprinted in Louis Poyman (ed.), The Theory of Knowledge, Third edition, pages 22-39.  See page 24.  Compare the translation found in Michael Huemer’s Epistemology – Contemporary Readings, page 517: “I shall never get out of the habit of confidently assenting to these opinions, so long as I suppose them to be what in fact they are, namely highly probable opinions–opinions which, despite the fact that they are in a sense doubtful, as has just been shown, it is still much more reasonable to believe than to deny.”)

        Similarly, Keith Lehrer formulates his argument in terms of the concept of knowledge, and he also says that he assumes that "if a man knows that p, then he is completely justified in believing that p"  (Ibid., page 57).  So Lehrer's argument is explicitly concerned with skepticism concerning knowledge.

(2) Sometimes, however, an author may advance an argument in support of one conclusion, but it turns out that the argument, if sound, also supports another, stronger conclusion.  So it's important to ask whether, even though the arguments of Lehrer and Descartes are explicitly concerned with skepticism regarding knowledge, they couldn't be deployed in support of a stronger conclusion.

        In the case of Descartes, the appeal seems to be to the fact that certain things are possible, and while the fact that something is possible - e.g., that one is dreaming or hallucinating - may well show that one is not justified in being certain that something is the case, it's not at all clear how a mere possibility can show that something is not likely, or even that it is not highly likely.  So it doesn't look as if Descartes' lines of argument can be extended in any straightforward way to the case of justified belief.

        What about Lehrer's arguments?  Initially, it might seem that the same will be true as in the case of Descartes' arguments - given that the notion of being completely justified seems to play a central role in Lehrer's argument.  But I think that appearances may be deceiving here.  For in arguments that Lehrer offers in sections VIII and IX of his paper, he advances arguments that have the following structure:

(1) One is not justified in holding that certain skeptical hypotheses are false unless one can offer a reason for thinking that they are false.

(2) One cannot appeal to one's ordinary beliefs in support of the claim that the skeptical hypotheses are false, since those ordinary beliefs are only justified if the skeptical hypotheses are false.  Appeal to beliefs that one ordinarily thinks are justified would therefore be question begging.

(3) If one is not justified in holding that the skeptical hypotheses are false, then one is not justified in being completely certain regarding any beliefs that are incompatible with the skeptical hypotheses.

But this argument for the conclusion that one is not justified in being completely certain regarding any beliefs that are incompatible with the skeptical hypotheses appears to be arrived at by a stronger claim - the claim, namely, that until the skeptical hypotheses are shown to be unjustified, our ordinary beliefs cannot be assumed to be even justified, let alone completely justified.

(3) As regards the explicit target, the skepticism of David Hume is a much more thoroughgoing skepticism than that of either Descartes or Lehrer, since it is directed not merely against knowledge, but against justified belief.

(4) In addition, and as we shall see later in this course - in connection with topic IX on the justification of induction - Hume's skepticism is also more thorough-going than skeptical positions that challenge the claim that one can have justified beliefs about the physical world and other minds.  For Hume argues that one cannot have justified beliefs even concerning one's own future experiences.  (Consider the choice between Berkeley's view of the world and a view according to which physical objects exist independently of any minds.  One might think that there was no way of deciding between those two hypotheses concerning reality, but think that one could at least know that one's experience would be as it would be if either were true.  But Hume wants to say that not even that belief is justified:  one's present and past experiences do not justify any conclusions concerning one's future experience.)

2.  One Basic Skeptical Pattern of Argument

3.   Possible Responses to this Basic Skeptical Pattern of Argument

        Possible responses to the above skeptical argument against justified beliefs can be seen as directed against various steps in the argument.  The main possibilities are as follows:

(1) Direct Realism.  This is the claim that one does have non-inferentially justified beliefs concerning the subject matter in question.

(2) Reductionism.  This is the view that, ultimately, there is no semantical gap between the type of propositions that constitute one's evidence and those that constitute the desired conclusion.  (Illustrate with other minds and physical objects:  logical behaviorism and phenomenalism.)

        Given a reductionist analysis of the relevant statements, the idea then is that one may be able to move either deductively, or via instantial generalization, from one's evidence to one's conclusion.

(3) Instantial Induction without Reductionism.  This is the view that even though statements concerning one's conclusion cannot be analyzed in terms of the types of statements that describe one's evidence, one can nevertheless arrive at inferentially justified beliefs of the relevant sort by means of instantial generalization from one's evidence.  (Illustration:  Other minds and the argument from analogy.)

(4) The Explanatory Theories Approach:  Hypothetico-Deductive Method.  The basic claim here is that there is a legitimate method of non-deductive reasoning other than instantial generalization that can carry one from one's evidence to the desired conclusions.  This method is variously labeled "the method of hypothesis", "hypothetico-deductive method", "inference to the best explanation", etc.

4.  Possible Responses in Different Areas?

4.1 The Justification of Beliefs about Physical Objects

        In the case of beliefs about physical objects, three of the above responses have been vigorously pursued.  First, a number of present-day philosophers defend direct realism, and hold that at least some beliefs about physical objects and events can be noninferentially justified, so that there is no gap between evidence and conclusion that one needs to bridge.

        Secondly, during the middle part of the 20th Century, and earlier, many philosophers adopted a reductionist view of physical objects, and held that propositions about physical objects could be analyzed in terms of propositions about sensory experiences.  This approach - which is known as classical phenomenalism - is, however, not widely adopted today, since, as we shall see, it is exposed to some very strong objections.

        Thirdly, a number of present-day philosophers defend the idea that beliefs about physical objects are inferentially justified on the basis of beliefs about sensory experiences, where the inference is held to be a matter of an inference to the best explanation:  the postulation of a world of physical objects that interact in certain ways, and that causally give rise to experiences, is the hypothesis that best explains various patters and regularities in our sensory experiences.  (This is the position known as the representative theory of perception, or as indirect realism, or causal realism.)

4.2 The Justification of Beliefs about Other Minds

        In the case of beliefs about other minds, three main approaches have pursued.  First - and, as in the case of phenomenalism, especially during the middle part of the 20th Century - many philosophers adopted a reductionist approach known as logical (or analytical) behaviorism, according to which propositions about mental states, and about the mind, can be analyzed in terms of propositions about behavior - both actual behavior, and dispositions to behave in relevant ways.  Since the behavior in question was thought of as behavior that necessarily involved physical movement, if logical behaviorism were correct, then justifying beliefs about the mental states of others would be no more difficult than justifying beliefs about the movements, etc., of physical objects, and so the problem of other minds would be nothing more than a case of knowledge of physical objects.

        Logical behaviorism is open to some very strong objections, however, and, as a result, has large been replaced by the view that mental states, rather than being nothing more than behavior, both actual and possible, are, instead, contingently identical with neurophysiological events.  This view also makes the problem of other minds less difficult than it would otherwise be, for although one does not in general observe the relevant neurophysiological events, one can argue that one can justify the belief that events of the relevant sort do occur via an inference to the best explanation.

        The problem of other minds becomes much more difficult, however, if one rejects both logical behaviorism and central state materialism, and holds, instead, that experiences involve properties that are not reducible to the properties that are postulated by physics.  Then one has to appeal to the claim that one knows, in one's own case, that such properties as experiencing the color red, or enjoying the taste of vegemite, or the smell of lilacs, do exist, and then arguing that the similarities that one sees in the physical makeup and behavior of others justifies one in believing that similar experiences are present in the case of others.

        What form does such an argument take?  One natural answer is that it is a matter of generalizing - via instantial generalization - from one's own case, and then applying those generalizations to other bodies, in order to arrive at the conclusion that there are also experiences associated with those other bodies.  But one can also view the relevant reasoning, as we shall see, as involving an inference to the best explanation.

4.3 The Justification of Beliefs about the Past

        In the case of other minds, and in the case of physical objects, reductionism is an option that has certainly exercised considerable appeal, at least in the past, if not today.  Is reductionism a serious option in the case of knowledge of past events?  It would seem not.  For what sort of reductionist account could one offer in the case of past events?  The only possibility, it would seem, would be to hold that propositions about the past are to be analyzed in terms of propositions about the present and/or the future - so that, for example, to say that there were dinosaurs might be analyzed as saying that certain sorts of bones, etc., now exist.  This view has been adopted by some philosophers - such as Jan Lukasiewicz and Arthur Prior - but it seems like a very implausible view, and it also seems that there are very strong objections to it:  for one thing, it implies that there are no causes of present events, since it entails that the only thing that really exists is the present state of the world.

        If reductionism is set aside, one is left with two main options with regard to how one might justify beliefs about past events.  First, there is direct realism, which says that at least some beliefs about the past - certain memory beliefs - are noninferentially justified.  Secondly, there is the possibility of an inference to the best explanation, in which one attempts to show that the best way of explaining the memories that one has is by the hypothesis that certain events really happened, and have played a role in bringing about the current state of the world, including one's memory beliefs.

4.4 The Justification of Beliefs about the Future

        Finally, what about beliefs about the future?  The main thing to notice here is that the general pattern of skeptical argument set out above does not work in the case of beliefs about the future unless it works in the case of beliefs about the past.  For if one can show that beliefs about the past can be justified, then one will be able to have justified beliefs to the effect that a was an event of type P, and that a was followed, at a certain temporal distance D, by an event of type Q.  Given a number of justified beliefs of that general form, and no justified belief to the effect that k was an event of type of type P that was not followed at a temporal distance D by an event of type Q, one will be able to use instantial generalization to conclude either that it is likely that every event of type P is followed, at a temporal distance D, by an event of type Q, or, more modestly, that it is likely that the next event of type P that one is justified in believing exists will also be followed by an event of type Q.

        The upshot is that if skeptical challenges to beliefs about past events can be answered, then so can a skeptical challenge of the sort that we are considering here to beliefs about the future, and the skeptic who wishes to challenge the claim that one can have justified beliefs about the future will then have to mount a deeper skeptical argument - namely, one the questions the legitimacy of instantial generalization itself.  But this, of course, is precisely what David Hume did.

        First, note the interrelatedness of questions of justification and questions of analysis.

        Secondly, and as we have just seen, different responses may be appropriate to challenges in different areas.  Indeed, some responses that are promising in some areas may be completely unavailable in the case of challenges in other areas.

5.   The Skeptical Argument and Theories Apparently Involving Reference to Unobservable Entities

        Even if one can somehow give a satisfactory account of the justification of beliefs about ordinary, macroscopic physical objects, a precisely parallel problem arises with respect to how we can be justified in believing in theories that postulate things that are too small to be observed - such as, perhaps, electrons, protons, and neutrons, or even smaller entities, such as quarks.  With respect to this latter question, some of the same options will arise as in the case when the skeptical argument is applied to ordinary beliefs.  In particular, one of the main options is reductionism - which here will be the view that, e.g., statements about sub-atomic particles can be analyzed in terms of statements about macroscopic objects - while another crucial option is the appeal to some type of inductive reasoning that is not a matter of instantial generalization.

6.   A Very Different Type of Skeptical Argument

        The type of skeptical argument we have been considering to this point attempts to establish a very strong conclusion, since it attempts to show not only that we are not justified in accepting various non-skeptical conclusions  – such as that there is an external, mind-independent world, or other minds, or past events – but also that those hypotheses have no prima facie credibility beyond their a priori probability of being true.  Thus it is claimed, for example, that the sense experiences we have provide no support at all for the existence of an external, mind-independent world, on the ground that there is no legitimate form of reasoning by virtue of which it can be shown that the existence of those sense experiences makes it more likely that there is an external world than it would be if one had no sense experiences at all.

        There is a very different type of skeptical argument that attempts to establish instead a significantly weaker conclusion – namely, that although there are considerations that do make it the case that non-skeptical hypotheses have likelihoods of being true that are greater than their a priori probabilities, each such likelihood is either (1) less than one half – so that the relevant non-skeptical hypothesis is more likely to be false than to be true – or else (2) the likelihood in question is equal to one half, so that the skeptical hypothesis in question is as likely to be true as the non-skeptical hypothesis, or else (3) the probability of the non-skeptical hypothesis being true is only marginally greater than one half, so that there is a really significant chance that skepticism, say, concerning the existence of an external, mind-independent world, or concerning other minds, or concerning the past, or concerning the future, or concerning theoretical entities, is true.

        But although the conclusion of the different sort of skeptical argument that I have in mind is more modest, I think that this other sort of skeptical argument is ultimately more threatening, because it is much more difficult to answer.

        I shall not set out this second type of argument in a detailed way at this point.  It will be better to do that, I think, when I attempt to refute skepticism, since my refutation will be geared to that argument.  But here, in very brief outline, is the essential idea.