Friday, December 11, 2020

On Self-Defeating Skeptical Arguments

Usually, self-defeating arguments are bad. If say "Trust me, you shouldn't trust anyone", my claim (you shouldn't trust anyone), if true, undermines the basis I've offered in support (that you should trust me). Whoops!

In skeptical arguments, however, self-defeat can sometimes be a feature rather than a bug. Michel de Montaigne compared skeptical arguments to laxatives. Self-defeating skeptical arguments are like rhubarb. They flush out your other opinions first and themselves last.

Let's consider two types of self-defeat:

In propositional self-defeat, the argument for proposition P relies on a premise inconsistent with P.

In methodological self-defeat, one relies on a certain method to reach the conclusion P, but that very conclusion implies that the method employed shouldn't be relied upon.

My opening example is most naturally read as methodologically self-defeating: the conclusion P ("you shouldn't trust anyone") implies that the method employed (trusting my advice) shouldn't be relied upon.

Since methods (other than logical deduction itself) can typically be characterized propositionally then loaded into a deduction, we can model most types of methodological self-defeat propositionally. In the first paragraph, maybe, I invited my interlocutor to accept the following argument (with P1 as shared background knowledge):

P1 (Trust Principle). If x is trustworthy and if x says P, then P.
P2. I am trustworthy.
P3. I say no one is trustworthy.
C. Therefore, no one is trustworthy.

C implies the falsity of P2, on which the reasoning essentially relies. (There are surely versions of the Trust Principle which better capture what is involved in trust, but you get the idea.)

Of course, there is one species of argument in which a contradiction between the premises and the conclusion is exactly what you're aiming for: reductio ad absurdum. In a reductio, you aim to prove P by temporarily assuming not-P and then showing how a contradiction follows from that assumption. Since any proposition that implies a contradiction must be false, you can then conclude that it's not the case the not-P, i.e., that it is the case that P.

We can treat self-defeating skeptical arguments as reductios. In Farewell to Reason, Paul Feyerabend is clear that he intends a structure of this sort.[1] His critics, he says, complain that there's something self-defeating in using philosophical reasoning to show that philosophical reasoning shouldn't be relied upon. Not at all, he replies! It's a reductio. If philosophical reasoning can be relied upon, then [according to Feyerabend's various arguments] it can't be relied upon. We must conclude, then, that philosophical reasoning can't be relied upon. (Note that although "philosophical reasoning can't be relied upon" is the P at the end of the reductio, we don't accept it because it follows from the assumptions but rather because it is the negation of the opening assumption.) The ancient skeptic Sextus Empiricus (who inspired Montaigne) appears sometimes to take basically the same approach.

Similarly, in my skeptical work on introspection, I have relied on introspective reports to argue that introspective reports are untrustworthy. Like Feyerabend's argument, it's a methodological self-defeat argument that can be formulated as a reductio. If introspection is a reliable method, then various contradictions follow. Therefore, introspection is not a reliable method.

You know who drives me bananas sometimes? G.E. Moore. It's annoyance at him (and some others) that inspires this post.

Here is a crucial turn in one of Moore's arguments against dream skepticism. (According to dream skepticism, for all you know you might be dreaming right now.)

So far as I can see, one premiss which [the dream skeptic] would certainly use would be this: "Some at least of the sensory experiences which you are having now are similar in important respects to dream-images which actually have occurred in dreams." This seems a very harmless premiss, and I am quite willing to admit that it is true. But I think there is a very serious objection to the procedure of using it as a premiss in favour of the derived conclusion. For a philosopher who does use it as a premiss, is, I think, in fact implying, though he does not expressly say, that he himself knows it to be true. He is implying therefore that he himself knows that dreams have occurred.... But can he consistently combine this proposition that he knows that dreams have occurred, with his conclusion that he does not know that he is not dreaming?... If he is dreaming, it may be that he is only dreaming that dreams have occurred... ("Certainty", p. 270 in the linked reprint).

Moore is of course complaining here of self-defeat. But if the dream skeptic's argument is a reductio, self-contradiction is the aim and the intermediate claims needn't be known.

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ETA 11:57 a.m.: I see from various comments in social media that that last sentence was too cryptic. Two clarifications.

First, although the intermediate claims needn't be known, everything in the reductio needs to be solid except insofar as it depends on not-P. Otherwise, it's not necessarily not-P to blame for the contradiction.

Second, here's a schematic example of one possible dream-skeptical reductio: Assume for the reductio that I know I'm not currently dreaming. If so, then I know X and Y about dreams. If X and Y are true about dreams, then I don't know I'm not currently dreaming.

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[1] I'm relying on my memory of Feyerabend from years ago. Due to the COVID shutdowns, I don't currently have access to the books in my office.

8 comments:

Arnold said...

If Mr. Moore's" dream skepticism" is up to date, then could you clarify his understanding of here now...as he mentioned it in the link...

The "Sextus Empiricus" link pursues non-no beliefs toward everything, even time...and goes on to say...for-tranquility also...
...What is time and what is tranquility are the questions there...

Thanks for the reads...

SelfAwarePatterns said...

Interesting post, as always Eric!

It seems like one way to break out of self defeat loops is to talk in terms of probabilities rather than absolutes. So if I say, you shouldn't trust anyone *completely*, including me because I might be wrong on *some* things, then a skeptical point is made without going down in a circular loop.

Similarly, admitting that philosophy can be right on some things, but its conclusions are far from reliable knowledge, allows for an observation of that unreliability without contradiction.

A reductio ad absurdum works if it demonstrates an actual logical contradiction. But far too often they only demonstrate a conclusion that strongly violates our intuitions. The problem is its been demonstrated many times that, by the standards of our intuitions, reality is absurd.

Mike

Arnold said...

The relativity of skepticism can-be certainty in a moment" of time...
...what does Socrates want, what is Socratic value...

Eric Schwitzgebel said...

Thanks for the comments, folks!

Mike: "A reductio ad absurdum works if it demonstrates an actual logical contradiction. But far too often they only demonstrate a conclusion that strongly violates our intuitions." Right, that would be a reductio ad falsum if the intuitions are correct, or ad absurdum if we stipulate that they are correct as a premise in the argument. However, as you say, "The problem is its been demonstrated many times that, by the standards of our intuitions, reality is absurd." Indeed, this is the this of the book I'm currently writing!

Callan said...

Hello Eric,

You don't think it's just poisoning the well? I mean it's kind of like relying on people to trust vaccines and to take the shot to then put out a vaccine to that people will take a shot of that makes those people not trust vaccines.

Why wade in with wording that expects to be trusted to begin with?

Callan said...

Erg, sentence structure was a bit rough, sorry - to try again: It being like relying on people to trust vaccines enough to take the vaccine shots...to then go and take a vaccines that makes people not trust vaccines.

Mike Roche said...

Very interesting post!

One comment on a point in your addendum:

“ First, although the intermediate claims needn't be known, everything in the reductio needs to be solid except insofar as it depends on not-P. Otherwise, it's not necessarily not-P to blame for the contradiction.”

It doesn’t really matter “what’s to blame”, right? Even if not-P is “not to blame”, one can still confidently conclude that P deductively follows from the premises. Given those premises, not-P can’t be true, and so, given those premises, P must be true,

Eric Schwitzgebel said...

Right, thanks for the clarification, Mike! To the extent "blame" could potentially come apart from proof of the contradiction, it's irrelevant.