The skepticism I’m speaking of here is that sort that denies that we can know whether the world exists, whether any of our theories are true, whether other people exist, whether the moment before now really occurred and whether the moment after now will occur, and so on. Let’s call this sort of skepticism “decadent skepticism”. The decadent skeptic wins every argument. I’m not referring to the sort of highly valuable skepticism we find in Derrida, Lyotard, certain moments of Wittgenstein, Latour (yes, I think he’s a huge skeptic), Hume, Nietzsche, Zizek, etc., where suspicion is called on a variety of our claims about reason, what we can know, what morality is about, etc. These latter anti-philosophers are absolutely vital to the philosophical project of discerning how far we can get with reason (without having to make appeals to gods and whatnot). I’ll call this latter sort of skepticism “critical skepticism”. This skepticism, I think, does real service to thought and society by revealing biases, prejudices, assumptions, and illicit forms of power behind claims and institutions that claim to be acting on behalf of the good, public welfare, and the true.
The problem with decadent skepticism is that it makes no difference. I guess here my assumption is that good philosophy makes a difference in how we do things. Good philosophy leads you to see different things than you would have otherwise seen, pursue the formation of different types of institutions, regard different things as being ethical obligations, etc. Good philosophy makes a difference in what we believe to be of value, how we judge, how we cognize, how we build, and so on. Decadent skepticism ultimately makes no difference (at least as far as I can tell).
Suppose we’re unable to refute the extreme skeptic in his claim that the world does not exist, that there are no other people, that we’re just a brain in a vat hooked to electrodes, that yesterday really did not exist and tomorrow will not happen, etc. The decadent skeptic always wins his arguments because he’s undermined any possibility of evidence through his skepticism. The thing is, though, my life wouldn’t be any different were all of this true. I’d still have to behave as if there are cause and effect relations, as if tomorrow will happen, as if yesterday I placed myself in debt, as if there are other people, as if I need to work in order to eat, etc. This is why I call extreme skepticism decadent skepticism. It is “philosophical” in the worst vernacular sense of the term: that sense which wonders about things that ultimately make no difference whatsoever. So no, I have no idea how to refute the extreme skeptic, but nonetheless neither my life nor that of the skeptic’s is any different if he’s right.