Are you sure you have hands?

Massimo Pigliucci in Scientia Salon:

Skepticism is a venerable word with a panoply of meanings. When I refer to myself as “a skeptic,” I mean someone inspired by David Hume’s famous dictum: “In our reasonings concerning matter of fact, there are all imaginable degrees of assurance, from the highest certainty to the lowest species of moral evidence. A wise man, therefore, proportions his belief to the evidence” [1]. Or, as Carl Sagan famously phrased it, “Extraordinary claims require extraordinary evidence” [2]. Oh, and if there is one thing I resent it is being mislabelled as a “cynic,” meaning a naysayer with no sense of humor…

But skepticism (and cynicism, for that matter!) in philosophy is much, much older than that, and has at the least a couple of additional meanings [3]. According to so-called (by Sextus Empiricus, second or third century CE, [4]) “academic skeptics” (because they belonged to Plato’s academy, post-Plato), such as Carneades (214-129 BCE) [5], we cannot have any epistemically interesting knowledge. A different type of skeptic, the Pyrrhonian (named after Pyrrho, 365–ca 275 BCE) denied even that we can deny the possibility of knowledge, a meta-skepticism, if you will. Few modern philosophers are interested in Pyrrhonism, while academic skepticism has a long and venerable tradition, including perhaps most famously Descartes’ “radical doubt” thought experiment, in which he imagined a Machiavellian demon determined to trick him about what he thought he knew. Descartes then asked whether it would be possible, under those circumstances, to actually know anything at all. His answer, of course, was in the affirmative, and took the form of his famous cogito, ergo sum (I think, therefore I am) [6].

There is, of course, a much more fun way to think about the problem of skepticism in epistemology, and that is by using the 1999 scifi move The Matrix as a philosophical thought experiment [7].

More here.