An excellent conference on Concepts and Conceptual Analysis here at the ANU has just finished. Lots of interesting talks -- David Braddon-Mitchell on fine-graining two-dimensionalism (into three-dimensionalism!) to handle a priori equivalent but cognitively distinct concepts, Laura Schroeter and John Bigelow on Against Apriori Reductions (arguing against Chalmers and Jackson 2001, by appealing to an "improv" model of concepts), and Anna Wierzbicka on her remarkable project of conceptually analyzing all linguistic expressions into 65 or so conceptual primitives that can be found in all languages. Also I gave a talk on a different approach to primitive concepts (no paper yet, but here's a Powerpoint version).
Here I'll say a few words about Frank Jackson's talk, which set out his version of two-dimensionalism and made clear a number of differences with my version. Frank complained that people sometimes assume that he believes everything I believe (except about consciousness), so it's useful to clarify the differences.
A first, relatively predictable difference is that Frank doesn't like versions of two-dimensionalism that conceive of the two dimensions as operating over two different modal spaces (epistemic possibilities and metaphysical possibilities, say) -- he wants a single space of worlds, with at most a difference in centering.
A second difference is that Frank holds that while linguistic expressions have two-dimensional content, mental states do not -- they only have primary intensions (A-intensions), not secondary intensions (C-intensions). I think this is tied to Frank's endorsement of a "map" theory of belief, on which there aren't really token beliefs, and certainly nothing like sentences in a language of thought (it may also be tied to his view of the special role of linguistic conventions in determining content). I'm more sympathetic with token beliefs than Frank is, but I think that even if belief states were holistic map-like states, one could still associate these with individuals and properties represented, and so with entities like secondary intensions, in a way that's useful for many explanatory purposes (e.g., for tracking certain sorts of agreement and disagreement across believers, and certain sorts of communication). The alternative view has some odd results: e.g. no relevant mental content will be shared by two people who believe I am hungry and He is hungry of the same individual, although intuitively these people are agreeing and in some sense believe "the same thing".
The third difference, and the most surprising to me (well, the second would have been just as surprising, but I knew about it already) was Frank's suggestion that there aren't really two different "true at a world" relations between sentences and worlds, but just one. Now one might think that the existence of two different relations here is a sine qua non for two-dimensionalism (at least for versions with a single space of worlds). But what Frank meant by this was that the "true at a world considered as actual" relation (i.e. the primary intension or A-intension relation) is derivative on the ordinary "true at a world" relation. His view was something like the following: sentence S is true at world W considered as actual iff a related sentence S' is true at W simpliciter. For example, when S is 'the actual F is phi' (and F is "one-dimensional", introducing no 2D variations of its own), S' is 'the F is phi'. Frank suggested that this model generalizes: all "two-dimensional" expressions are equivalent to expressions of the form 'the actual F' for some one-dimensional F, and their two-dimensional evaluation can be defined in these terms. (Aficionados will note the resemblance to Davies and Humberstone's framework, where 2D evaluation is defined entirely in terms of the behavior of 'actually').
This claim surprised me in a few ways. First, the claim that all names, natural kind terms, and indexicals are equivalent to expressions of the form 'the actual F' is a strong and substantive assumption, one that I've always taken it that the two-dimensionalist isn't committed to. The claim looks especially difficult in the case of indexicals such as 'I'. Frank suggested that 'I' is equivalent to something like 'the actual utterer of this sentence', but then one has the further indexical 'this sentence' to worry about, and so on. (In discussion afterwards Frank said he might want to have a somewhat different treatment for evaluation of indexicals.) And for names and natural kind terms, the relevant F is at least not easy to find (especially given the requirement that F be one-dimensional).
Further, it seems to me that we test hypotheses about whether N is equivalent to 'the actual F' in part by considering scenarios as actual ("what if the world turned out this way?"), and seeing whether the two expressions give equivalent results (as in e.g. the standard 2D construal of what's going on in Kripke's 'Godel' argument). But this suggests that we have a grip on evaluation of expressions in worlds considered as actual that doesn't derive from our grip on associated descriptions. Of course one could hold that an underlying description is tacitly guiding our evaluation. But my view has always been that a two-dimensionalist needn't make this claim, and that we can evaluate expressions in worlds considered as actual simply by reasoning about those worlds (e.g. by what one should infer if one accepted the hypothesis that W is actual), whether or not there is an associated description.
What emerged from the discussion is that for Frank, two-dimensionalism is a sort of byproduct of a prior descriptivism. On his view there is 2D-independent reason to think that names and the like are equivalent to rigidified descriptions, and once we have this equivalence we can use it to ground a 2D analysis. On my view, by contrast, things are closer to the other way around. We have an independent grip on how to evaluate sentences in worlds considered as actual and counterfactual (grounded in our grip on certain epistemic and subjunctive modal notions), and this grounds a 2D analysis of content. This analysis might then be used to ground a sort of quasi-descriptivism, or at least a framework that resembles descriptivism in some respects by delivering some of the results results that descriptivists wanted. But the descriptivism plays no essential role in grounding the two-dimensionalism, and some of the stronger claims of descriptivism need not be true (for example, I don't think that names are semantically equivalent to expressions of the form 'the actual F'). I think that in addition to being correct, this version is more effective dialectically, in that to presuppose descriptivism is to presuppose a view that a lot of people think is false.
In any case, it's good to get clear on the alternatives. The difference is in some respects analogous to that between Frege's and Russell's views in the philosophy of language. As Kent Bach notes, although both Frege and Russell are sometimes described as descriptivists about names, only Russell is a true descriptivist who thinks that names are abbreviations of definite descriptions. Frege thinks that names have non-extensional semantic values, senses, which are tied to epistemic role and which are in certain respects like the senses of descriptions. One might say that Frank's two-dimensionalism is "Russellian" while mine is "Fregean".