Philosophical skepticism is notoriously difficult to refute. I am not speaking of popular skepticism, as defined by magazines such as Skeptical Inquirer and Skeptic. These are only skeptical of certain things, generally are not skeptical of the issues they support, and hold that knowledge is possible.
Philosophical skepticism is more rigorous, holding that ultimately we can’t know anything. Ancient Greeks such as Pyrrho of Ellis took skepticism to a rigorous level, doubting whether we can know anything, including whether or not we are doubting. Radical skepticism presents questions such as “It might be that we are a brain in a vat, being fed sensory data by an alien via a complex machine.” Or “it could be that we are halucinating” or “It could be we are being deceived by an evil demon.” We can’t just say that this is absurd, for the mental game here is to logically refute it, which is notoriously difficult. All the skeptic has to do is say “It could be an illusion” and leaves us with the problem of positively refuting it beyond all doubt. As silly as this may seem at first glance, otherwise intelligent people have based their lives on the basis that no one can be sure of any meaning. The comedian Steve Martin went into comedy because he could not find anything meaningful in life, and felt that no one is able to be sure of anything meaningful in life.
So we are presented with a gnarly problem, similar to the Buddhist pantheist, who says that all sensory data is an illusion.
But there are some good refutations. First, all claims that we can’t know ultimately are self-refuting, saying that we know we can’t know. Any attempts to show this self-refutation wrong ends in either another self-refutation or nonsense. And nonsensical claims cannot be responded to, for they make no claim.
Second, all skeptics appeal to sensory data to claim that we can’t trust sensory data, which is another self-refuting claim. Skeptics say we can’t trust sense data, yet they use sense data to try to prove their case that we can’t trust sense data.
Third, author Peter Klein in his book Certainty claims skepticism can be absolutely refuted if the following can be proven: 1) There is no good reason for believing that knowledge of p is always false, and 2) there is good reason for believing that knowledge of p is sometimes true. Thus, meaning is established, at least some of the time. Fourth, all claims that we might be deceived due to dreaming or by halucinating imply that we cannot tell the difference between those things and reality. But there is a fundamental sense difference between dreams and reality, or else we wouldn’t have two words to name them. If we can’t tell the difference between dreaming and reality, how is it that we have made a distinction enough to use two different names? There must be some fundamental distinction between them, and we must be able to tell them apart, or else we wouldn’t have two names with two meanings.
Ultimately, the radical skeptic cannot make a claim that holds up. Skepticism is refuted, and absolute knowledge is established.