Here's the problem I'm having: for a long time I have been a smug philosophical foundationalist and realist. Whenever I encountered someone firmly situated in that cluster of existentialism, pragmatism, and skepticism which tends to briefly captivate the young and cynical, I would throw out one of the usual self-righteous rejoinders: "If the world doesn't really exist, why are you bothering to argue the claim with me?" "If there is no truth, then in what sense are you committed to the belief that 'there is no truth'?" "If no belief about reality is more valid than another, then what intellectual tools do you have to resist Nazism or some other manifestation of evil?" etc. Pretty good, right? These are all downright stumpers. But the trouble is that punching holes in someone else's philosophical system cannot alter the position of what Antony Flew called the "well-girded skeptic." In fact, it only serves to confirm skepticism. Showing the wobbliness of a particular formulation of existentialism or pragmatism or what-have-you can only ever tell you about that formulation-- it can never tell you that your own preferred belief system, whatever it may be, is any better.
I did not always realize this. I think early on in college I regarded schools of philosophy as something like political parties. You pledge allegiance to them based on what you "agree with" (for preexisting reasons) in their platform. But as Kant remarks somewhere, there cannot really be any distinctions between materialism and idealism, realism and skepticism. There are only philosophical truths as they have so far been established, and philosophical errors. You don't get to pick and choose among them. Because of this-- it is often the case that the differences in the "schools" boil down more to semantics than ideas. The materialist who says she knows a chair is real because she has sense experience of it is functionally saying the same thing as the idealist who says: the sense experience you have described is simply what we mean by the word "chair."
Similarly, I'm not sure there are any real differences between myself and the existentialist. It is no good to ask the existentialist the questions I listed at the outset unless I can answer them myself. In other words, it is no good being a foundationalist unless I can adduce some foundations. But if you ask atheist realists like me, or religious traditionalists, or post-post-modernists, or whatever, why we are not willing to go the whole hog in the skeptical direction, you will end up with mere credos, doxologies, statements of faith. In my case it would run something like this: I know I can't provide foundations for morality, for my belief in external reality, etc. But I still believe in these things (and so, probably, do you) because on the deepest level of my being I feel them to be true, and I can't really imagine thinking straight, eating, chewing, walking around, or breathing without them. The Christian might say the same thing but add God and a few other doctrines to the list.
But all of this, presented here as a response to existentialism, is in fact simply a restatement of the existentialist position. The whole point of the existentialist position is that it allows one to make commitments of this sort even in the absence of foundations. We are all in the same boat, then, cast on the same "dark ocean, without shores or lighthouses, strewn with many a philosophic wreck." This seems a more jolly picture to me, actually, than the position in which we started, drawn up like hostile armies-- but it means forgoing future acts of self-righteousness on my part where existentialists are concerned.
Of course, there are still refutations to be made. The horn-rimmed, goateed ironist we know from college or grad school who blithely insists that "there is no truth" contradicts his own claim by the very act of making it, and we are still free to delight in pointing this out to him. But we should always remember that our own claim-- that there must, by definition, be truth out there somewhere, but we will never have access to it, and we are stuck with a set of belief statements that we regard as true despite their lack of foundations, is not really all that different from his. We are all existentialists, even those of us who loath the term.
Thoughts for the comment thread, gentle reader? (I use the singular there for more than rhetorical reasons.)