Your argument is about as valid as "Take it on faith". Unless appealing to pragmatism, your argument is circular in using the belief of others when you can't justifiably assume their existence. Second, your argument is irrational in that it appeals to "Everybody believes X" to support X. Thirdly, a source claiming X to be so is only evidence for X being so if you have reason to consider the source reliable.
You are also mixing up "epistemic order" with "empirical order", to frame two new concepts. "Epistemic order" represents orders of inference- if I infer A from B and B from C, then C is prior to B and B is prior to A in epistemic order regardless of the real-world relation of whatever they are. "Empirical order", of course, represents what is the empirical cause of what (if indeed anything causes anything).
A person detects their own thoughts in a different way from the way they detect their own senses, so they are unrelated in epistemic order. You raise a valid point about assuming that one's thoughts really are one's thoughts, but unless resorting to the Memory Argument (which is part of the Evil Demon argument I discussed) they are at least avaliable as arguments to consider.
The Foundationalist skeptic is arguing that believing in the existence of the world IS IRRATIONAL. Without resorting to the arguments I describe in the first post, there seems to be no way to get around this. Pragmatics clearly isn't one, after all.
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.