Thinking you have figured out how to solve age old philosophical problems, very quickly, is generally a bad sign.
Nonetheless, ever since TFing Intro Epistemology last semester, I find myself feeling more and more that worries about external world/other minds/memory skepticism involve an incoherent melange of a sharp proof theoretic question, together with a fuzzy normative question.
The proof theoretic question is something like:
- Can you prove the external worlds exist, starting from premises that contain only necessary truths?
- Can you prove memory is reliable starting from premises containing only necessary truths and true statements about current experience?
[Where "prove" can be cashed out in various formal ways - e.g. first order logic, or modal logic, or intuitionistic logic - to yield different variants of the question.]
And the normative question is:
When is it empistemically OK to assume premises in a given set X, given that I cannot prove them (in logic L) from premises in set Y?
Once we've made this distinction, and noted that some premises which one might assume are true, and others false, the normative question looses much of its interest (at least for me).
Furthermore, we can point out to the skeptic who e.g. believes in the reality of past experiences but not in the external world, that his position appears exactly analogous to our own. We can challenge the skeptic to provide any kind of distinction between what's OK to assume vs. not OK to assume that looks remotely principled enough to motivate our revising our judgments on the subject.
"In what sense," we can say to the skeptic, "do you know that e.g. there are infinitely many primes, or that it's impossible to know things about the external world, such that I don't also (by those very same standards) count as knowing that I have a hand?. In both cases, there are more radical skeptics whom we cannot persuade. Thus, in saying that you know, but I do not, you seem to be just stomping your foot and making the unmotivated value judgment that it's OK to assume what you assume and not OK to assume what I assume.
Why should I be more confident that you have correct moral beliefs about what it's OK to assume, than that I have correct descriptive beliefs about whether I have a hand?"