All right, here's where I think we are (please correct me if I'm wrong):

You advocate skepticism.  However, since your belief amounts to "no belief is better or worse than any other," you cannot give any good reasons for skepticism, or say that it's better justified than the empiricism I support.

At one point, you did appear to try to make arguments as to why skepticism makes sense... but now we're at a point where we realize that, to make arguments as to why skepticism makes sense is to implicitly agree that belief is not arbitrary.

In other words, defending or arguing for your skepticism is self-contradictory.

More than arguing, we also now realize that for you to feel like you have good reasons for your own positions is also self-contradictory.  In an earlier response, you appeared to make it sound like your skepticism was based on your making sound judgements...

Through definition and logic: when considering the question, "How can I know?", I would answer "Because of...", but when I examined that because of, I realized it contained propositions that I didnt know for certain, either.

...but you've insisted that judgements, as such, have no basis or merit.

And so now, in "support" of skepticism, we simply have this:

I don't know why I have it, I just do OR I don't havea  choice in the matter

In a nutshell:

I am an empiricist.  I insist that I have good reasons for my belief in empiricism, and that your skepticism is incorrect and should be abandoned.  You have no way of disagreeing with me about any of that, because 1) according to you, my beliefs are as good as any other, including your own; and 2) any argument that you might make would be an admission on your part that you have the capacity to make sound judgements, which is consistent with my worldview but contrary to your own.

You are a skeptic, but you feel that you have no good reason to be one--that your beliefs are, in fact, totally arbitrary--and therefore you can't explain why they make sense, or whether they even do.

Am I close?

***

Regarding "certainty," I believe that, taken together with your belief in skepticism, it is a meaningless concept.  Allow me to explain why.

This is our exchange from earlier:

Let us say that I claim that the sky is blue.

I have sensed data, I have made a judgement.  You claim that my judgement is flawed in that it is inherently "uncertain."

What would "uncertain" mean in this context?  Would "uncertain" mean that it is possible that the sky is not blue?  That it might actually be pink?  Or taupe?  (Or that there may be no sky at all?)

If we say that no person can trust their own judgements, then there is no way for anyone to ever establish that the sky is not, in fact, blue.  There are no means by which a person could ever know that I was wrong, either now or in the future, and therefore no reason to suspect that I might be wrong.

"Uncertainty" and, moreover, "being wrong" would become meaningless.

They become meaningless because no one could ever say that the sky was pink, taupe, or anything other than what I've said it is; the only way that "uncertain" makes sense is if we posit that there are conditions under which I might be wrong--that the sky could, in fact, be some color other than blue, and that some person could conceivably be in a position to know it.

But if there are conditions under which I could be wrong, justifying a lack of certainty, then those conditions will assume that non-arbitrary judgements can be made.

In short, by insisting that all judgement is arbitrary, you make the distinction between "certain" and "uncertain" arbitrary as well--if no one could ever tell the difference, it's just as well that we consider the sky "certainly" blue.

As Joel would say to the Mads, "What do you think, Sirs?"

@ postscript:  well, "belief set", anyway. 

A.  Fair enough, although 3, 4, and Matrix are not compatible with the posited belief set, and 2 was not intentionally within it although it could have been interpreted that way easily enough.  (And he did in hypothetical fact change his belief of what the Moon IS made of, whatever he thinks of what it WAS made of. )

I agreed, way back, that "senses can't ABSOLUTELY PROVE that a worldview is the true really for real truth", and it now looks like you've agreed that a belief set can be contradicted by sense data.  So I think it was all a misunderstanding that you thought I was saying belief sets could somehow yield absolute truth beyond themselves, right? 

B.  You know I'm not saying practicality is a method or reflection of absolute truth, right?  Just that acting on sense data as perceived is the only practical path since you can't actually do anything else.

I'd ask what else is proposed, but I'm scared

A. I think your missing my point. If it is true that at every point you can choose from a number of possible beliefs on sense data and that they are all equally possible - then whatever scenario you're developing here is irrelevant to the problem.

I see you creating a set of beliefs to forcefully contradict itself at another point and I don't know why, since you don't have to contradict yourself. You've got an infinite number possibilities to choose from at any point, and whether you pick coherent ones or contradictory ones the problem still remains that each one involved had no reason for being picked.

That's why the doctrine of empiricism isn't a working method of truth. It doesn't point to any one interpretation of sense data. It is offers them all and leaves you in recognition that by picking any one you're being arbitrary and so, therefore, not engaging in a method of truth.

In case there is still further confusion on something I suspect: You can't presuppose sense data is only evidence of one thing. Why? Because just by merely examining possibilities we see that it is evidence of many things. So if you were trying to tell me that you could simply presuppose that a certain blob of sense data only represented cheese... that goes in the face of this unavoidable admission that it simply doesn't.

B. I see "acting on sense data as perceived" as inseparable from the implication that you know something or know something to interact with in a certain way. That's some truth. And again, there's no way to establish these, now termed, 'perceptions' and there corresponding 'actions'. 

By skepticism, I take it you mean the idea that one denies everything because there is no proof of anything.  In that case, no, I'm not a skeptic, because a denial is just as much a position as an advocacy.  What I am saying, however, is that any statement I make about reality would have to be an assumption.  My recognition of that would not be contradictory because it's not a statement about reality: it's one of definition.  I do not say you cannot use logic or make judgements, but that you cannot make judgements on reality with certainty.

The uncertainty in your exampe would be that we do not know if it's actually the sky that eminates the blue, or if there is even a sky.

What I mean by uncertainty is assumption; I do not know if the sky exists, and I cannot know, and to say it does or doesn't would be an assumption. The only thing that is certain (not an assumption) is the existence of sense data; what it means or represents is completely unknown, of course.

If that didn't really clear anything up, see my previous response to Final;  I may have misunderstood your last part.

A.  Except, in the case of the belief sets that include that belief ... then it does.  This was all, entirely, SOLELY about my assertion that the "any" in "any belief set is supported by sense data" is not correct.  Only infinite numbers are. 

B.  Think about it.  The only actions you can reliably* take are the ones you perceive the results of, yes?  So empiricism is the only belief that lets you realistically* do anything.  Thus the only "practical" one, thus the most "practical" one. 

*I do not by these words imply that you actually KNOW "really for real" that you are in ACTUAL FACT doing these actions.  I mean rather that your ONLY MEANS OF INPUT replies to your output in a fairly internally consistent way.  (BTW, this includes dreams to the extent that the inputted world explains the discrepancy.)  And that "doing" anything requires an input/output system.  And that "practicality" by definition refers to "doing" things. 

Do you now see my justification for saying something is practical even if we don't know that it is true?

I've been in thread death marches before.  But never, I think, one with quite so many mirages. 
(Also, when will the page turn over??)

I reject "definition" as being a category outside of "reality." :)  I think reality is bigger, and fairly all-encompassing.  Or, if it's not, then how do you know that definition is something different from reality?  How can you be certain of it?

But yeah, my contention is that, whether you call it "denial" or whatever, your statements are "just as much a position as an advocacy."  After all, I'm saying that you are advocating a position--the position of skepticism.

If you're uncomfortable with the label "skeptic," that's fine, though I don't see why you should be.  Skepticism is a philosophical position with a long and proud tradition (I just happen to believe it to be wrong).  Check this out, as I think it mirrors many aspects of our conversations:

http://www.galilean-library.org/manuscript.php?postid=43798

Or, there's this: (It becomes fairly dense, but you might at least give it a read until the author brings up The Matrix. )

http://plato.stanford.edu/entries/skepticism/

This one is possibly the clearest:

http://skepdic.com/skepticism.html