(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, and the substantive skepticism of Keith Lehrer are directed (at least explicitly) at knowledge claims. Thus Descartes, in the first chapter of his Meditations, 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." (Pojman, page 42)
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" (Pojman, page 61). 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.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.
(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.
(3) As regards the explicit target, the skepticism of David Hume is a much more through-going 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.)
(1) If a belief is justified, it is either non-inferentially justified, or inferentially justified.
(2) It is not possible to have non-inferentially justified
beliefs concerning the truth of the propositions in question.
Thus, one cannot be immediately, or non-inferentially justified in
believing that there are other minds, or an external physical world, or
that certain past events have taken place, or that certain future events
will take place.
(3) That gap between the evidence and the desired conclusion cannot be bridged by means of any deductive argument.
Two considerations can be offered in support of this claim. In the first place, it can be argued that the semantical content of the relevant evidence, in each of those cases, differs from the semantical content of the beliefs that one is attempting to base upon the evidence in question. For the beliefs that one is putting forward as one's evidence seems to refer to different things than the beliefs that one is trying to justify. One's evidence consists, the skeptic will argue, of beliefs about one's own present, mental states, whereas beliefs about an external world, or about other minds, or about the past, of about the future, are not beliefs about one's own mental states - or at least, not about one's own present mental states.
Secondly, and even more forcefully, one can show that no deductive bridge is possible by appealing to possible worlds in which the beliefs that constitute one's evidence would be true, but the beliefs that one is attempting to justify would not be. Thus, in the case of beliefs about physical objects, one can appeal to brain-in-vat scenarios. In the case of other minds, one can appeal to the idea of possible worlds where other humans are automata, or else puppets controlled by alien beings. In the case of beliefs about the past, there is the possibility that the world came into existence only a moment ago, with apparent memories, etc. in place. Finally, in the case of beliefs about the future, one can appeal to the idea that the world is about to be annihilated, or to drop out of existence. So the truth of the beliefs that constitute one's evidence does not suffice to guarantee the truth of the conclusion, and the inference, therefore, cannot be a deductively valid one.
(4) The only possible logical relations that there can be between premises and conclusion are (i) deductive relations, and (ii) inductive relations.
(5) The only acceptable form of inductive reasoning is instantial generalization. That is to say, induction is always a matter of arriving at some generalization on the basis of instances of that generalization. (So, for example, to establish that all ravens are black, one needs either to establish it by appealing to instances - that is, by appealing to the fact that one is justified in believing that there are particular objects, - a, b, c, d, ... such that a is black and a is a raven, b is black and b is a raven, c is black and c is a raven, d is black and d is a raven, ..., and that there is no object k such that one is justified in believing that x is a raven and x is not black - or else one needs to be able to deduce it from other generalizations that have been established in that way.)
(6) In all of the four cases we are considering, instantial generalization cannot be used to justify the beliefs in question. The reason is that, for example, in order for instantial generalization to allow one to justify some particular claim about the existence of a physical object, one would have to establish some generalization that involves reference to physical objects. But if a generalization involves references to physical objects, then any instance of it must also refer to at least one physical object. There is, therefore, no way to get started, since until one has some knowledge of the sort one is attempting to justify, one has no instances that one can start from.
(7) Consequently, there is no way that one can get from the premises to the desired conclusion by means of inductive reasoning.
(8) Hence the semantical gap that exists between one's evidence and the conclusion that one would like to justify inferentially on the basis of that evidence cannot be bridged either deductively or inductively, and therefore the beliefs in question cannot be inferentially justified.
(9) Therefore, one cannot have any justified beliefs concerning other minds, the physical world, past events, or the future.
(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.
(2) 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.