Skeptics usually argue that from the fact that (1) it is possible that you are in certain situations that I will call 'skeptical scenarios' where you would not know anything you believe by processing sensory information, it follows that (2) you do not know anything that you believe by processing sensory information no matter what circumstances you are in. Skeptical scenarios, which range from Descartes' deceitful demon to the modest Gettier case, are situations such that: if you were in one of them, (a) your sensory information would not be any different from the way it would be if you were not in it, and (b) some (often most) of what you believe as a result of this information might be false, even though you have that information.
I point out the assumption on which the skeptic's argument rests and argue that it is false, namely, CH': If, through process P, S were to arrive at the belief that p in circumstances C in which p is true and S failed to know p, and if in different circumstances C' (in which p is also true) S arrived at the belief that p through P, then S would not know that p in C' either. I also argue that Nozick's important explanation of skepticism in Philosophical Explanations (which does not attribute CH' to the skeptic) is not right since it assumes that the following Principle of Closure is false: If S knows that p and S believes that q by deducing it from p, then S knows that q.
Document Object Identifier (DOI)
University of Southern California
Luper-Foy, S. (1984). What skeptics don't know refutes them. Pacific Philosophical Quarterly, 65(1), 86-97. doi:10.1111/j.1468-0114.1984.tb00215.x
Pacific Philosophical Quarterly