Am I missing something obvious? Why should we accept that the conclusion is true, rather than discarding one of the premises?
For example, there is an actual argument in the philosophy of science that is based on the supposition that there is a substance that is "identical to water in every respect" but "is not H2O but rather XYZ".
Philosophers take this "argument" quite seriously, and get rather testy when you point out that the premise is a contradiction: if something has "all the properties of water", then that must include properties under electrolysis, and under electrolysis water decomposes to 2H and O, not X, Y and Z (whatever those might be). If you say this to a philosopher they will reply, "Yes, but what if it doesn't?" as if that was a coherent statement.
So the answer to your question is, "Because if we allowed ourselves to discard one of the premises we'd have a much harder time motivating this whole ridiculous discussion, and since our livelihood depends on such ridiculous discussions, we're not about to let that happen!"
Even here, the issues are related to metaphysics, not so much about chemistry:) There is a jump from empirical world (the world we are in) to possible worlds. Here is the article: Is water necessarily identical to H2O, by David Barnett http://spot.colorado.edu/~barnetdb/my_papers/Barnett_Water.p...
So saith the Amalgamated Union of Philosophers, Sages, Luminaries, and Other Professional Thinking Persons, eh?
The point is that even IF you give Mary all that information, she's still missing some information. Then, that information must not be physical information. More to the point, there's a difference between the points you presented: In (A), you say Mary has all _physical_ knowledge (Jackson uses the word information, but the difference is unimportant). In (B), you note that she gets new _knowledge_. It follows that this knowledge must be non-physical, since we assumed that she had all physical knowledge, and yet, she didn't have this new knowledge.
Experience is information. It doesn't make sense to imagine a situation in which she has all the information about a thing, except experience. Either she has all of the information about it, or she doesn't.
Jackson's point here is specifically to show that there are at least two kinds of information: physical information, and information that is not physical. Bearing that in mind, what do you mean by: > "Either she has all of the information about it, or she doesn't."
I suppose that my objection falls under 4.2 in http://plato.stanford.edu/entries/qualia-knowledge/, specifically, metaphysical physicalism: "there are no non-physical individuals, properties or relations and no non-physical facts".
What would be a good example of non-physical information?
Perhaps the point here is that "Mary gains new knowledge" is supposed to be so manifestly obvious that it doesn't need to be argued? If so, this seems far from obvious to me.
That's more what confuses me about the structure of this argument, though - I would take the experiential insight to be more "physical". So I still don't understand what they mean by physicalism.