i. douven - 2005 - lewis on fallible knowledge
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Lewis on Fallible Knowledge
Igor Douven
Department of Philosophy, Erasmus University Rotterdam
Abstract
Lewis has offered a contextualist epistemology that he claims is non-fallibilist.
The present note aims to show that, while there seems to be a simple argument
for Lewis claim, the argument is fallacious, and Lewis epistemology is fallibilist
after all.
Contrary to what many philosophers hold, Lewis [1996] claims that fallibilism is mad-
ness (p. 550).1 Not that he has any deep argument to offer against the position. To
speak of fallible knowledge, Lewis thinks, just soundscontradictory (p. 549):
If you are a contented fallibilist, I implore you to be honest, be naive, hear it
afresh. He knows, yet he has not eliminated all possibilities of error. Even if
youve numbed your ears, doesnt this overt, explicit fallibilismstillsound wrong?
(p. 550)
I strongly doubt that these words will do much to persuade the faithful fallibilists. But
for now let us grant that there is something odd-sounding to overt, explicit fallibil-
ism (the significance of the qualification will become apparent shortly), and that wehave reasonto whatever precise extentto reject a theory of knowledge if it counte-
nances fallibilism of this sort. Then we have reason to reject Lewis own contextualist
theory of knowledge. For although explicitly meant to be non-fallibilist, it does not
escape (overt) fallibilism after all. So I will argue.
The centerpiece of Lewis theory is the following definition of knowledge: Sknows
that P iffSs evidence eliminates every possibility in which not-P . . . except for those
possibilities that we are properly ignoring (p. 554). What makes the theory contex-
tualist is that it depends on the context in which a knowledge attribution is made, or
being evaluated, whether a possibility is properly ignored. Exactly how context sorts
possibilities into those that are and those that are not properly ignored, Lewis specifies
by means of seven rules, which can be usefully classified as permissive and prohibitive
rules, respectively. The permissive rules are defeasible and tell us what possibilities
we may properly ignore; the prohibitive rules circumscribe the possible defeaters. An
example of a permissive rule is the Rule of Reliability, according to which we may
defeasibly properly ignore possibilities concerning error in reliable processes (such
as perception and memory; p. 558). An example of a prohibitive one is the Rule of
Actuality, according to which the possibility that is actual is never properly ignored
(p. 554 f). The rule that will be most crucial to our concerns is another prohibitive
rule, the Rule of Attention; according to this rule, a possibility not ignored at all is
ipso factonot properly ignored (p. 559).
1All page references given in parentheses are to Lewis [1996].
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A word on the notion of possibility: Lewis does not require that possibilities be
maximally specific; they just must be specific enough, where a possibility counts as
specific enough if it cannot be split into subcases that differ in some epistemologicallyrelevant way (p. 552). To borrow an example from Hawthorne [2002:243], the possibil-
ity that there is a dog in front of one does not qualify as a specific enough possibility
in Lewis sense, for it can be split in, for instance, a subcase in which one hallucinates a
dog while there is a dog before one anyway and a subcase in which the dog causes the
visual impression; from an epistemological viewpoint these are relevantly different
situations of course. Accordingly, such generally characterized possibilities as the
possibility that not-P and the possibility that Sis wrong about P fail to qualify as
sufficiently specific not-Ppossibilities in Lewis sense.
Already from the above outline it is evident that on Lewis theory a person can be
truly said to know even if she has not eliminated all possibilities of error, namely if
these possibilities are properly ignored in the context in which the knowledge attribu-
tion is made. Thus in one sense Lewis theory is undeniably fallibilist. But this is a sort
of fallibilismwhich we may label meta-linguistic fallibilismthat Lewis apparentlyregards as harmless and that at any rate he sees no need to oppose. In speaking about
Lewis theory, and specifically about what knowledge attributions people can make
under what circumstances on that theory, we are not ourselves attributing knowledge
to anyone. A fortiori, then, we are not attributing fallible knowledge to anyone. And
what Lewis abhors are attributions of fallible knowledge, utterances in which we as-
cribe knowledge to a person but in the same breath assert that the person might be
wrong. We in effect can take Lewis
He knows, yet he has not eliminated all possibilities of error (F)
to define this sort of fallibilism, as follows: an epistemology is overtly fallibilist iff,
according to it, (F) can, under some circumstances or in some contexts, be truly as-
serted. Notice that the difference with meta-linguistic fallibilism concerns assertions
of (F): meta-linguistic fallibilists do not claim that (F) never can be true; what they
claim, or at least what Lewis claims, is that it never can be truly asserted.
And prima facie Lewis seems to have successfully realized his intention to steer
clear of overt fallibilism. After allthe argument goesas soon as we mention any
uneliminated possibilities of error, we are attending to them, and thus, by the Rule of
Attention, are no longer properly ignoring them. By Lewis definition of knowledge, an
attribution of knowledge must then be false. Thus the mention of the uneliminated
possibilities of error in the second part of (F) makes false the knowledge claim made
in the first part of that sentence (supposing there are any uneliminated possibilities
of error; if not, the sentence is false anyway). Hence, on Lewis theory, (F) can never
be truly asserted. Or can it? I shall now argue that, indisputable though it may seem,
the foregoing little argument is fallacious, and Lewis is committed to overt fallibilismafter all.
Suppose one of our conversation partners utters the following sentence:
Shas not eliminated all not-P possibilities. (A)
Is there a sense of attending to a possibility in which this utterance should suffice to
have us attend to any uneliminated not-Ppossibilities? To attend to something means
to give heed to it, be aware of it, have it before ones mind. After hearing (A), we may
be expected to consider, or entertain, the proposition it expresses, that is, have some
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attitude toward that proposition (e.g., believe it, or doubt it, or wonder whether it is
true). Thus, arguably, the utterance will bring before our minds the proposition that
there exist not-P
possibilities uneliminated byS
s evidence. However, on the under-standing of possibilities explicated three paragraphs back, that proposition does not
qualify itself as an uneliminated not-P possibility, and the question was whether the
utterance of (A) ought to bring before our minds any such possibility.
The answer, plainly, must be negative, if only because (A) may be false, in which
case there isno uneliminated not-Ppossibility that could be brought before our minds.
More interestingly, it even seems hard to maintain that the utterance of (A) should
suffice to bring before our minds any uneliminated not-Ppossibilities if (A) is true. To
see why, consider that (A) may be both truly and sincerely asserted without even the
asserter having before her mind such possibilities. For instance, she may remember
from an epistemology course she once took that for all propositions like Pthere exist
so-called skeptical possibilities that cannot be eliminated by any amount of evidence.
Even when pressed, however, she might not be able to recollect a single one of those
possibilities. That is to say, she may know that uneliminated not-Ppossibilities existbut, at least at present, not have the faintest idea what they are like and therefore
cannot in any reasonable sense be said to have them, or some of them, before her
mind. Now, if (A) can be truly and sincerely asserted without even the speaker having
before her mind any not-Ppossibility, then why should hearing that sentence suffice
to bring before the listeners minds such possibilities? It would thus seem that utter-
ances of (A) are compatible with no person in the context of utterance attending to
any uneliminated not-P possibilities.
If correct, that spells trouble for our little argument against overt fallibilism, for
that clearly requires that there be some sense in which we are attending to unelimi-
nated not-Ppossibilities after the utterance of (A). According to that argument, after
all, it isbecause(A) is a constituent of
Sknows that P, though she has not eliminated all not-P possibilities (B)
that anyone who utters (B) or hears (B) being uttered must be attending to, and thus
cannot be ignoring and, ipso facto, cannot be properly ignoring, some uneliminated
not-Ppossibilities (provided there are any). Is there any way in which the argument
can be saved from a threatening quick refutation?
The best response I can think of goes something like this:
The notion of attending is not an altogether precise one. While it seems
correct that a person who entertains the proposition expressed by (A) need
not have before her mind any particular not-Ppossibilities, and thus need
not be attending to such possibilities inthatsense of attending to a pos-
sibility, she still may be said to be attending to them in some indirect,mediate wayvia the proposition that there exist not-Ppossibilities, so to
speak. To be a bit more precise, we might say that an uneliminated not-P
possibility is indirectly attended to by a person S iff it makes true the
proposition that there exist uneliminated not-P possibilities and Senter-
tains that proposition. The contrast, of course, is withdirectly attending
to a possibility, which does require that Shave that possibility before her
mind. So, while in the above example the utterance of (A) is compatible
with no person attending directly to any uneliminated not-Ppossibilities,
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it should suffice to have all such possibilities be attended to indirectly by
all participants to the conversation.
Preliminary to assessing the effectiveness of this response, let me note that thedistinction between possibilities directly and indirectly attended to is orthogonal to
that between possibilities that are and those that are not specific enough in Lewis
sense. In particular, it would be wrong to think that a possibility that is attended to
only indirectly must thereby fail to be specific enough. A speakers assertion of (A)
may hold true in virtue of the skeptical possibilities typically discussed in epistemol-
ogy courses, and surely these count as specific enough, whether or not the speaker
has any of them before her mind. Of course, if she is attending to them indirectly,
via the proposition that there exist such possibilities, she may be unable to specify
any one of them. But whether a possibility is specific enough, that is, whether it is in-
divisible in epistemically relevant subcases, is one thing; whether, or to what extent,
it is specified in a given context, or even specifiable by any of the speakers in that
context, is quite another. Further note that the fact that a possibility may be beforeones mind more or less clearly does not obfuscate the directly/indirectly attending
distinction, as a possibility that is attended to indirectly may notand, if it is attended
to indirectly only, will notbe before ones mind in any intelligible sense of having
something before the mind.
Grant that the directly/indirectly attending distinction makes sense. Then that
still only enables the Lewisian contextualist to ward off a first stab at the argument
against fallibilism. For now the question arises whether attending indirectly (in a
given context) to not-Ppossibilities uneliminated by a persons evidence is, on Lewis
theory, compatible with attributing knowledge ofPto that person (in the same con-
text). If it is, then the theory still does permit us to attribute fallible knowledge to a
personovertly and explicitly. For if while attending indirectly, but not directly, to
not-Ppossibilities uneliminated by a persons evidence I can truly say: She knows
that P, then surely I can also truly say: She knows thatP, yet she has not eliminated
all possibilities of error; the second conjunct of the latter sentence need not bring
anything to attention I am not attending to, nor need it make me attend differently
(quadirect/indirect) to anything I am attending to, when I say (truly, by supposition):
She knows that P.
Whether attending indirectly to not-Ppossibilities is compatible with the ascrip-
tion of knowledge ofPwill depend on whether possibilities that are attended to in-
directly, but not directly, can still be properly ignored, and thus, on what exactly is
implied by the Rule of Attention. Specifically, it depends on whether this rule can
plausibly be taken to imply only the first, only the second, or both, of the following:
(1) A possibility of error attended to directly is not properly ignored.
(2) A possibility of error attended to indirectly is not properly ignored.
We may suppose that it implies (1). Crucial for present purposes is only whether it
can also be held to imply (2). If Lewis epistemology is not to be committed to overt
fallibilism, then, as we just saw, the answer had better be yes.
It might seem that the fastest way to end this discussion would be simply to stipu-
late that, regardless of whether the Rule of Attention was meant to imply (2), from now
on it does. Unfortunately, however, this is not an option. Lewis theory is supposed
to be a contribution to descriptive epistemology; its aim is to provide an analysis of
the semantics of know. As for any such analysis, its adequacy is to be judged on
the basis of how well it accords with linguistic practice. Consequently, we will have
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to see whether there is any linguistic evidence indicating that the Rule of Attention,
when taken to imply (2), is part of the analysis of knowledge.
The Rule of Attention, says Lewis, was devised to account for a very particular typeof linguistic evidence: My Rule of Attention . . . [was] built to explain how the skep-
tic manages to sway uswhy his argument seems irresistible, however temporarily
(p. 561). The skeptic manages to sway us, Lewis thinks, by directing our attention to
some skeptical possibility or possibilities.2 Im willing to grant that this is so,3 and
that this gives support to the Rule of Attention under some interpretation of that rule.
The question is whether it supports the rule if that is taken to (also) imply (2).
Skeptics typically paint quite colorful pictures of the possibilities they suppose to
convince us that we can know nothing or next to nothing. We are, for instance, asked
to imagine that were brains in a vat, floating in a fluid of nutrients, our nerve endings
connected to a super-fast computer, the computer programmed in a certain specific
way, and so on and so forth. Whatever the exact details of such stories, they are
obviously meantand usually succeedto have us attend directly to some specific
uneliminated (and non-eliminable) possibility or possibilities of error; they do notjust try to make us aware that there are such possibilities. Hence, our (presumed)
responses to them at best provide support for the Rule of Attention interpreted as
implying (1).
There may well be evidence for the Rule of Attention from sources other than
our responses to the skeptic. I shall now argue, however, that if this rule is taken to
imply (2), then linguistic evidence rather seems to indicate that it is notamong the
rules governing our usage of know.
Ironically, a first indication in this direction comes from the practice that moti-
vated the Rule of Attention. Skeptics typicallypaint colorful pictures of uneliminated
possibilities of error, I said. As far as I know, however, this is how theyinvariably
proceed.4 Be that as it may, it is certainly true that no skeptic has ever tried to con-
vince us of her position just by stating that, however good our evidence, we might bemistaken about virtually anything we claim to know, that is, by merely directing our
attention to the proposition that for virtually all of our knowledge claims there exist
possibilities of error not eliminated by our evidence. Yet if the Rule of Attention im-
plied (2), that should work. To appreciate this, note that from Lewis theory it follows
that, for almost any contingent proposition P, if in some context we can truthfully
attribute knowledge ofP to a person S, we must in that context be ignoring certain
not-Ppossibilities not eliminated by Ss evidence. That means that if in such a context
someone were to say to us: Despite all your evidence, there are still uneliminated
possibilities of error for just about any proposition you claim to know, that would
have us consider the true proposition that there are uneliminated possibilities of error
for virtually any proposition we claimed to know and, consequently, would make us
2
Lewis seems to have in mind here what Williams [1999] terms the Cartesian skeptic; next to this sortof skeptic, Williams identifies anAgrippan skeptic. Very roughly, the latter argues that any attempt to
justify a claim must eitherlead to an infinite regressorto dogmatismorto circular reasoning, and that
therefore the possibility of knowledge cannot be maintained. Since at least prima facie her argument
does not seem to make any appeal to skeptical possibilities, it may well be that concessive responses
to the Agrippan skeptic cannot be explained by invoking the Rule of Attention. Note, however, that the
Agrippan skeptical argument has force only if justification is a necessary condition for knowledge, and
this Lewis denies (p. 551). So presumably he would say that those who are persuaded by that argument
are simply misguided about what is required for knowledge.3But see Feldman [1999], Oakley [2001], and Rysiew [2001] for alternative explanations of the way we
tend to respond to the skeptic.4That is to say, this is how Cartesian skeptics seem to invariably proceed; see note 2.
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attend indirectly to a non-empty class of uneliminated possibilities of error for most
of them. If, then, the Rule of Attention were to imply (2), these possibilities would
no longer be properly ignored and we could, accordingly, no longer truly ascribe theknowledge to ourselves we previously did. Hence, it would follow that merely men-
tioning that we might be wrong about virtually anything we claim to know suffices to
make false virtually all attributions of knowledge we made or would otherwise make
to ourselves.
It is certainly telling that skeptics typically go to great lengths in arguing for their
position where, if the Rule of Attention implied (2), they could achieve their goal
so easily. As intimated, however, this is only anindication against the truth of the
supposition that the rule implies (2), not conclusive evidence against it; skeptics, after
all, might concoct their fantastical stories merely for rhetorical or literary purposes.
Better evidence doesnt seem hard to come by, though. For consider that we simply do
not findas surely we ought to if the Rule of Attention implied (2)that just raising
the possibility that a person is mistaken about a given proposition, without giving
the least specification of a scenario that could explain how she might have come topossess the evidence she possesses were the proposition false, generally suffices to
make people reluctant to attribute knowledge of that proposition to her or to make
them withdraw a previous attribution of knowledge of the proposition. Suppose,
for instance, you tell someone you know that Gore lost the 2000 elections, and that
person says: You might be wrong there. Would there be anything improper or out
of the ordinary if you replied: Well, I might, but I dont think I am? It seems not.
I rather suspect that, were you to retract your knowledge claim in response to the
other persons utterance, most people would take that to indicate that the claim was
not sincerely made in the first place, or perhaps that you suffer from some form of
character weakness. In any event, we will not normally consider such a retraction as
being a simple consequence of the semantics of knowwhich it would be if a rule
implying (2) were among those governing our usage of that verb.I am not saying that we alwaysare willing or able to maintain an earlier knowledge
claim once our attention has been pointed to the proposition that there are possibili-
ties of error not eliminated by our evidence. Usuallyjust saying that I might be wrong
will not suffice to make me retract a claim to the effect that I know the first moon land-
ing was in 1969. In some situations, however, like for instance a situation in which my
life, or (less dramatically) my winning a large prize, depends on not falsely asserting
when the first moon landing occurred, mentioning that possibility just mightsuffice.
But evidence showing that undersomecircumstances we do retract knowledge claims
after the mentioning that there are uneliminated possibilities of error is utterly un-
helpful to Lewis, for, it will be remembered, overt fallibilism makes no stronger a
claim than that (F) sometimes, insomecontexts, can be truly asserted.
In sum, if the Rule of Attention is to be justified by reference to our usage of
know, then the rule must be taken to imply (1) at most. But if it does not im-
ply (2), then, it was argued, the rule cannot offer any help in preventing the attribution
of knowledge in a context in which the attributor merely entertains the proposition
that the subjects evidence fails to rule out some possibilities of error, and does not
have before her mind any particular uneliminated possibility of error. So in that case
the rule does not preclude that, in some contexts, (F) and other sentences similarly
attributing knowledge to a person while making one attend, but only indirectly, to
possibilities of error uneliminated by the persons evidence, can be used to make true
assertions. We must thus conclude that Lewis is committed not only to meta-linguistic
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fallibilism but also to overt fallibilism, madness or no.5
References
Feldman, R. [1999] Contextualism and Skepticism, Philosophical Perspectives
13:91114.
Hawthorne, J. [2002] Lewis, the Lottery and the Preface,Analysis62:242251.
Lewis, D. [1996] Elusive Knowledge, Australasian Journal of Philosophy 74:549
567.
Oakley, I. T. [2001] A Skeptics Reply to Lewisian Contextualism,Canadian Journal
of Philosophy31:309332.
Rysiew, P. [2001] The Context-Sensitivity of Knowledge Attributions,Nos 35:477
514.
Williams, M. [1999] Skepticism, in: J. Greco and E. Sosa (eds.)The Blackwell Guide
to Epistemology, Oxford: Blackwell, pp. 3569.
5I am greatly indebted to Tim Oakley for generously providing extremely valuable written comments
on an earlier draft; I am also grateful to Christopher von Blow and to two anonymous referees for
this journal for very useful suggestions for improvement. Versions of this paper were delivered at the
Universities of Amsterdam (Free University) and Leuven and at Erasmus University Rotterdam; I thank
the audiences on those occasions for their helpful questions and remarks.
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