Ok, response here from somebody who has studied philosophy. I disagree with a lot of what DSherron said, but on one point we agree - don't get a philosophy degree. Take some electives, sure - that'll give you an introduction to the field - but after that there's absolutely no reason to pay for a philosophy degree. If you're interested in it, you can learn just as much by reading in your spare time for FREE. I regret my philosophy degree.
So, now that that's out of the way: philosophy isn't useless. In fact, at its more useful end it blurs pretty seamlessly into mathematics). It's also relevant to cognitive science), and in fact science in general. The only time philosophy is useless is when it isn't being used to do anything. So, sure, pure philosophy is useless, but that's like saying "pure rationality is useless". We use rationality in combination with every other discipline, that's the point of rationality.
As for the OP's questions:
DSherron suggests following the method of the 13th century philosopher William of Ockham, but I don't think that's relevant to the question. As far as I can tell, ALL justificatory systems suffer from Munchausen's Trilemma. Given that, Foundationalism and Coherentism seem to me to be pretty much equivalent. You wouldn't pick incoherent axioms as your foundations, and conversely any coherent system of justifications should be decomposable into an orthogonal set of fundamental axioms and theorems derived thereof. Maybe there's something I'm missing, though.
DSherron's point is a good one. It was first formalised by the philosopher-mathematician Leibniz who proposed the principle of the Identity of Indiscernibles.
DSherron suggests that the LW sequence "A Human's Guide to Words" is relevant here. Since that sequence is basically a huge discussion of the philosophy of language, and makes dozens of philosophical arguments aimed at correcting philosophical errors, I agree that it is a useful resource.
You're right, my statement was far too strong, and I hereby retract it. Instead, I claim that philosophy which is not firmly grounded in the real world such that it effectively becomes another discipline is worthless. A philosophy book is unlikely to contain very much of value, but a cognitive science book which touches on ideas from philosophy is more valuable than one which doesn't. The problem is that most philosophy is just attempts to argue for things that sound nice, logically, with not a care for their actual value. Philosophy is not entirely worthl...
I have naturally read the material here, but am still not sure how to act on two questions.
1: I've been arguing out the question of Foundationalism v.s Coherentism v.s other similiarly basic methods of justifying knowledge (e.g. infinitism, pragmatism). The discussion left off with two problems for Foundationalism.
a: The Evil Demon argument, particularly the problem of memory. When following any piece of reason, an Evil Demon could theoretically fool my reason into thinking that it had reasoned correctly when it hadn't, or fool my memory into thinking I'd reasoned properly before with reasoning I'd never done. Since a Foundationalist either is a weak Foundationalist (and runs into severe problems) or must discard all but self-evident and incorrigible assumptions (of which memory is not one), I'm stuffed.
(Then again, it has been argued, if a Coherentist were decieved by an evil demon they could be decieved into thinking data coheres when it doesn't. Since their belief rests upon the assumption that their beliefs cohere, should they not discard if they can't know if it coheres or not? The seems to cohere formulation has it's own problem)
b: Even if that's discarded, there is still the problem of how Strong Foundationalist beliefs are justified within a Strong Foundationalist system. Strong Foundationalism is neither self-evident nor incorrigible, after all.
I know myself well enough to know I have an unusually strong (even for a non-rationalist) irrational emotive bias in favour of Foundationalism, and even I begin to suspect I've lost the argument (though some people arguing on my side would disagree). Just to confirm, though- have I lost? What should I do now, either way?
2: What to say on the question of skepticism (on which so far I've technically said nothing)? If I remember correctly Elizier has spoken of philosophy as how to act in the world, but I'm arguing with somebody who maintains as an axiom that the purpose of Philosophy is to find truth, whether useful or useless, in whatever area is under discussion.
3: Finally, how do I speak intelligently on the Contextualist v.s Invariantist problem? I can see in basic that it is an empirical problem and therefore not part of abstract philosophy, but that isn't the same thing as having an answer. It would be good to know where to look up enough neuroscience to at least make an intelligent contribution to the discussion.