There are two tripartite arguments I'd like to consider in tandem because I believe by looking at them in this way that we get a better appreciation of what is going on in each. Or, at least I think we better appreciate the kind of question that scepticism wants us to consider. Scepticism shouldn't be a matter of doubting what we know, whether locally or globally, but rather the arguments that we employ to show that scepticism is a possible position makes us think it less likely a position anyone may take up. I call this feeling of cognitive inertia: sceptical vertigo.
Back in 2006 or so, I presented a paper with that title to the Society for Sceptical Studies at the American Philosophical Association, Pacific Division meeting in Pasadena. I never did anything with the paper because I felt that it was merely repeating some of the same points of Stanley Cavell (The Claim of Reason, 1979), John McDowell ("Virtue and Reason," 1979) or Hilary Putnam ("Philosophy as the Education of Grown-Ups," 2006). However, Duncan Pritchard's recently released works ("Epistemic Vertigo," 2020; "Cavell and Philosophical Vertigo," 2021) have brought to light many new means of arguing for similar conclusions. In addition, it is good to see that Elijah Millgram's paper on a nearby cluster of issues has been published after many years of work ("Refuting Skepticism with Style," 2020).
When I was in graduate school, a colleague who had happened to have just finished his PhD on analytic epistemology used the term tripartite in a derogatory manner because he contended that the approach to skepticism using the tripartite argument was outdated and deeply flawed; I don't agree with him, if for no other reason than that the tripartite arguments make them presentable to specialists and non-specialists, alike. Likewise, they cut to the heart of the sceptical matter. One of the arguments is the argument from the sceptical hypothesis, and the other is (roughly) a Moorean response. Here are the arguments in schematic form:
Argument from the Sceptical Hypothesis
If I don't know that there is an external world, then I don't know that I have hands. (premise)
I don't know that there is an external world. (premise)
Therefore, I don't know that I have hands. (1,2)
(Roughly) A Moorean Response
If I don't know that there is an external world, then I don't know that I have hands. (premise)
I have hands. (premise)
Therefore, there is an external world. (1,2)
Both arguments are equally plausible, and all of the premises in each of the arguments are reasonably compelling. There is no reason to believe that I don't know that I have hands, and there is no reason to consider that the external world doesn't exist. If both arguments are equally plausible, have reasonably compelling premises, and are deductively valid, then it seems that both arguments could be true. One persons modus ponens is another's modus tollens, so they say.
Epistemologists who argue about scepticism claim that the two arguments are contradictory. They mean that we cannot uphold the conclusion of both conclusions, except upon pain of admitting a contradiction into one's own beliefs. Remember the law of noncontradiction, i.e., It is not the case that p and not-p is true, tells us that the conclusion of both arguments cannot be true.
It is not clear to me that one needs to accept a contradiction for the conclusion of both arguments to be true. The conclusion of the first argument is: I don't know that I have hands. The conclusion of the second is: There is an external world. The two don't seem to be contradictory. Note that in the first argument we have to assume that I don't know that there is an external world. Of course, there could be an external world, but my imperfect epistemic access to the external world would make me believe that any view that there is an external world is unjustified, even if true. That's a very different claim than to say that if I accept both of these premises and the valid deductive form of the argument that the conclusion must necessarily follow, i.e., that the argument is truth-preserving.
When we turn to the second argument, we find that an assumption is that I have hands. Again, this premise is consistent with the sceptical world in which the external world doesn't exist. My having hands in that world is just as likely as it would be in the case of an existent external world. Note that the assumption of the second argument doesn't contradict the conclusion of the first. That I have hands doesn't make it any more, or any less, likely that my justification for my not knowing that I have hands is unwarranted. The belief that I have hands and that I don't know that I have hands seem perfectly consistent.
Vertigo is a feeling of dizziness where either the external world is revolving around the individual (cited as "objective" vertigo) or the individual is revolving in space (cited as "subjective" vertigo). We cannot glom onto one or other of the arguments because once we do that, the other argument has refuted our position. The two positions provided in each of the above arguments seem equally likely. Also, I have shown that the conclusion of the two arguments needn't be understood to be contradictory. The best we get from the above examples is a sceptical vertigo. Hence, we will have to accept our sceptical vertigo in order to move on from these two arguments.
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