I'm not a positivist, and I don't argue like one. I think nearly all the arguments against the possibility of zombies are very silly, and I agree there's good prima facie evidence for dualism (though I think that in the final analysis the weight of evidence still favors physicalism). Indeed, it's a good thing I don't think zombies are impossible, since I think that we are zombies.

What's your reason for believing this?

My reason is twofold: Copernican, and Occamite.

Copernican reasoning: Most of the universe does not consist of humans, or anything human-like; so it would be very surprising to learn that the most fundamental metaphysical distinction between facts ('subjective' v. 'objective,' or 'mental' v. 'physical,' or 'point-of-view-bearing' v. 'point-of-view-lacking, 'or what-have-you) happens to coincide with the parts of the universe that bear human-like things, and the parts that lack human-like things. Are we really that special? Is it really more likely that we would happen to gain perfect, sparkling insight into a secret Hidden Side to reality, than that our brains would misrepresent their own ways of representing themselves to themselves?

Occamite reasoning: One can do away with the Copernican thought by endorsing panpsychism; but this worsens the bite from the principle of parsimony. A universe with two kinds of fundamental fact is less likely, relative to the space of all the models, then one with one kind (or with many, many more than two kinds). It is a striking empirical fact that, consciousness aside, we seem to be able to understand the whole rest of reality with a single grammatical kind of description -- the impersonal, 'objective' kind, which states a fact without specifying for whom the fact is. The world didn't need to turn out to be that way, just as it didn't need to look causally structured. This should give us reason to think that there may not be distinctions between fundamental kinds of facts, rather than that we happen to have lucked out and ended up in one of the universes with very few distinctions of this sort.

Neither of these considerations, of course, is conclusive. But they give us some reason to at least take seriously physicalist hypotheses, and to weight their theoretical costs and benefits against the dualists'.

One problem with this line of thought is that we've just thrown out the very concept of "experience" which is the basis of empiricism.

We've thrown out the idea of subjective experience, of pure, ineffable 'feels,' of qualia. But we retain any functionally specifiable analog of such experience. In place of qualitative red, we get zombie-red, i.e., causal/functional-red. In place of qualitative knowledge, we get zombie-knowledge.

And since most dualists already accepted the causal/functional/physical process in question (they couldn't even motivate the zombie argument if they didn't consider the physical causally adequate), there can be no parsimony argument against the physicalists' posits; the only argument will have to be a defense of the claim that there is some sort of basic, epistemically infallible acquaintance relation between the contents of experience and (themselves? a Self??...). But making such an argument, without begging the question against eliminativism, is actually quite difficult.