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” . Or, as Carl Sagan famously phrased it, “Extraordinary claims require extraordinary evidence” . 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 . According to so-called (by Sextus Empiricus, second or third century CE, ) “academic skeptics” (because they belonged to Plato’s academy, post-Plato), such as Carneades (214-129 BCE) , 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) .
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 . The movie famously begins with our hero, Neo, played by Keanu Reeves, living what he thinks is a perfectly normal life, which soon reveals itself to be anything but. Neo, turns out, is much closer to the famous “brain-in-the-vat” (BIV) scenario of modern philosophy of mind (to be precise, he is a body-in-the-vat), with all his “experiences” actually being fed to him via artificial stimulation for the purposes of an evil post-technological civilization of machines that have enslaved humanity.
There is a crucial scene in the movie  where Neo’s mysterious mentor, Morpheus (played by Laurence Fishburne) poses the question to Neo of whether he wants to keep living in the “reality” he knows, or if he has the guts to see “how deep the rabbit hole goes.” As we know, Neo chooses the red pill that characterizes the second choice and the movie unfolds from there.
Neo, of course, is initially (properly) skeptical (in the Humean sense) of what Morpheus is trying to convey. The latter might as well have asked his question along the lines of: “how do you know you are not a brain in a vat?” How would you answer that sort of question? Which is another way of asking: have we made any progress against (academic) skepticism?
My discussion here tracks the one put forth in Steup’s broader treatment of epistemology in the Stanford Encyclopedia of Philosophy . We begin with a quick look at the minimal version of the BIV argument:
(1) I don’t know that I’m not a BIV.
(2) If I don’t know that I’m not a BIV, then I don’t know that I have hands.
(3) I don’t know that I have hands.
This is a formally valid argument, i.e. its structure is logically correct, so any viable response needs to challenge one of its premises — that is, question what in logic is called its soundness. Before proceeding, though, we must note (as Steup does) that premise (2) is tightly linked to (indeed, it is the negative version of) the so-called Closure Principle: “If I know that p, and I know that p entails q, then I know that q” — a principle that is definitely eminently reasonable, at first sight. The application to our case looks like this: If I know that I have hands, and I know that having hands entails not being a BIV, then I know that I’m not a BIV. But — says the skeptics — the consequent of this “BIV closure” is false, hence its antecedent must be false too: you just don’t know whether you are a BIV or not!
There are, of course, several responses to the skeptic’s so-called “closure denial.” Steup examines a whopping five of them: relevant alternatives, the Moorean response, the contextualist response, the ambiguity response, and what one might call the knowledge-that response. Let’s take a quick look.
A first attack against the BIV argument is to claim that being a BIV is not a relevant alternative to having hands; a relevant alternative would be, for instance, having had one’s hand amputated to overcome the effects of disease or accident. This sounds promising, but the skeptic can very well demand a principled account of what does and does not count as a relevant alternative. Such an account could perhaps deploy a type of approach naturally enough called relevance logic , but that would get pretty technical, so I’ll leave it for another time.
Second attack: G.E. Moore’s (in)famous “I know that I have hands” response. This is essentially an argument from plausibility: the BIV goes through if and only if its premises (I don’t know whether I’m a BIV, so I don’t know whether I have hands) are more plausible than its conclusion (I don’t actually know whether I have hands). Which, of course, Moore famously denied — by raising one of his hands and declaring “here is one hand.” But why, asks (reasonably, if irritatingly) the skeptic? To make a long story short, Moore’s counter to the BIV argument essentially reduces to simply asserting knowledge that one is not a BIV. Which, ahem, pretty much begs the question against the skeptic .
Third possible anti-skeptic maneuver: the contextualist response. The basic intuition here is that what we mean by “know” (as in “I know that I have hands,” or “I don’t know that I’m not a BIV”) varies with the context, in the sense that the standards of evidence for claiming knowledge depend on the circumstances. This leads contextualists to distinguish between “low” and “high” standards situations. Most discussions of having or not having hands are low standards situations, where the hypothesis of a BIV does not need to be considered. It is only in high standards situations that the skeptical hypothesis becomes salient, and in those cases we truly do not know whether we have hands (because we do not know whether we are BIVs). This actually sounds plausible to me, though I would also like to see a principled account of what distinguishes low and high standard situations (unless the latter are, rather ad hoc, limited only to the skeptical scenario). Perhaps things are a bit more complicated, and there actually is a continuum of standards, and therefore a continuum of meanings of the word “know”? 
Fourth: the ambiguity response. Here the strategy is to ask whether the skeptic, when he uses the word “know” is referring to fallible or infallible knowledge. (This is actually rather similar to the contextualist response, it seems to me, though the argument takes off from a slightly different perspective, and I think is a bit more subtle and satisfying.) Once we make this distinction, it turns out that there are three versions of the BIV argument: the “mixed” one (“know” refers to infallible knowledge in the premises but to fallible knowledge in the conclusion), “high standards” (infallible knowledge is implied in both premises and conclusion), and “low standards” (fallible knowledge assumed in both instances). Once this unpacking is done, we quickly reach the conclusion that the mixed version is actually an instance of invalid reasoning, since it is based on an equivocation; the high-standards version is indeed sound, but pretty uninteresting (okay, we don’t have infallible knowledge concerning our hands, so what?); and the low-standards version is interesting but unsound (because we would have to admit to the bizarre situation of not having even fallible knowledge of our hands!).
Finally: the knowledge-that response, which is a type of evidentialist approach. The idea is to point out to the skeptic that the BIV argument is based on a number of highly questionable unstated premises, such as that it is possible to build a BIV, and that someone has actually developed the technology to do so, for instance. But we can deny these premises on grounds of implausibility, just like we would deny, say, the claim that someone has traveled through time via a wormhole on the ground that we don’t have sufficient reasons to entertain the notions that time travel is possible and that someone has been able to implement it technologically. Yes, the skeptics can deny the analogy, but the burden of proof seems to have shifted to the skeptic, who needs to explain why this is indeed a disanalogy. Can someone please get me a red pill?
Now, why on earth did we engage in this, ahem, academic discussion? Because I wanted to give you a flavor of how philosophy makes progress, and why it isn’t particularly fruitful to compare it with progress in the natural sciences (did you see any systematic observation or experiment peeking through the above?). Indeed, I am writing a whole book on this topic, which I will hopefully deliver to Chicago Press by the end of the summer. No, make that I will definitely deliver by the end of summer…
The idea is that philosophy is concerned with exploring conceptual, as distinct from empirical, spaces, which is precisely what we have done above. Indeed, you could go through it again and try to build a concept map  to see whether you followed the discussion correctly and to visualize its unfolding. The five responses presented by Steup can be thought of as five peaks in the conceptual space defined by the BIV problem, with other possible responses having been examined and discarded during the long history of the debate (those would be conceptual valleys, to continue the metaphor). Not all peaks are necessarily of the same height — where the height roughly measures how good a given response is, and even the precise position and shape of the peaks may vary over time, as philosophers keep refining them in response to counterarguments from the skeptics.
Moreover, the metaphor should make clear that even to ask the question of what is the true answer to the BIV problem is, in a fundamental way, to misunderstand the whole process. If the BIV question were an empirical one — like “how many planets are there in the solar system” — then it would have one definite answer , and a bunch of bad ones. But in conceptual space there often are several reasonable ways of looking at a particular problem (“answers”), and it will not be possible to pare them down to just one. (Another way to put this is to say that conceptual space is wider than, and underdetermined by, empirical space.)
So, what should we then make of (academic) skepticism and its critics? I think the value of the skeptical position is that it fosters epistemic humility: we are really not as smart as we often think we are, and in fact we don’t even have unequivocal answers to very basic questions about knowledge. As for the responses to the BIV problem, the five sketched above represent peaks of different heights in the proper conceptual landscape, and in my mind the ambiguity and the knowledge-that peaks are significantly higher than the rest, the Moore response is the lowest, and the relevant alternative and contextualist options are somewhere in the middle. But I’m sure we can have a discussion about that.
Massimo Pigliucci is a biologist and philosopher at the City University of New York. His main interests are in the philosophy of science and pseudoscience. He is the editor-in-chief of Scientia Salon, and his latest book (co-edited with Maarten Boudry) is Philosophy of Pseudoscience: Reconsidering the Demarcation Problem (Chicago Press).
 In An Enquiry Concerning Human Understanding, published in 1748.
 A dictum, by the way, which can easily and rigorously be formalized in Bayesian terms.
 Here is the obligatory SEP entry, by Peter Klein.
 On Sextus Empiricus.
 Here is my take on Carneades.
 On Descartes’ epistemology.
 You can do this and much more by engaging the fun essays in the collection put together by Susan Schneider, Science Fiction and Philosophy: From Time Travel to Superintelligence, Wiley-Blackwell, 2009.
 Here it is, for your viewing pleasure.
 Epistemology, by Matthias Steup, SEP, 2005.
 Relevance logic.
 On George Edward Moore.
 I know, I know, this is beginning to sound rather Clintonesque. Then again, the former President of the United States did study philosophy at Oxford as a Rhodes Scholar…
 On concept mapping.
 Well, kinda. The famous “demotion” of Pluto to “dwarf planet” hinges on the rather arbitrary — and in some sense philosophical — question of what counts as a planet and why. Incidentally, and contra popular perception, it was Caltech astronomer Mike Brown who was chiefly responsible for killing Pluto, not my friend Neil deGrasse Tyson. Neil did, however, write a popular book about the story.