2Dism and Epistemic Extension

We have been talking about two-dimensionalism in one of my seminars. One issue that came up was that of how names and kind-terms manage to refer (or "pick out something") in a (conceivable) scenario. Given two-dimensionalism, names refer (roughly) to their actual referents in metaphysically possible worlds but they refer to something that may be very different from their actual referents in (conceivable) scenarios. For example, 'David Chalmers' refers to David Chalmers in metaphysically possible worlds (where he exists) but in a given scenario, 'David Chalmers' may refer to someone who looks a lot more like David Lewis. So, 'David Chalmers = David Lewis' is metaphysically impossible but it may (in some contexts) be conceivable (for instance, if the speaker uses 'David Lewis' to mean, roughly, 'the author of The Plurality of Worlds' and uses 'David Chalmers' to mean, roughly, 'the author of The Conscious Mind').

Chalmers and Jackson say that if we have a complete canonical description of a scenario, then 'David Chalmers = David Lewis' is true there iff it is a priori implied by the description. The description (being canonical) cannot itself contain any names or kind-terms. But how then are we to understand the presupposed notion of apriority? If names and kind-terms are descriptions in disguise, then there is no problem seeing what is going on. But Chalmers and Jackson deny that this is so.

It's not that speakers associate descriptions with names and kind-terms (or that there necessarily are any such descriptions that could be associated with them). It's rather that if there is an implication of the aforementioned sort, then it corresponds to some (idealized) ability of the speaker in question. I found the following passages particularly illuminating (they are all from "Sense and Intension (SI)" or "Conceptual Analysis and Reductive Explanation (CARE)"):

"subjects are frequently in a position to identify the extension of a given concept, on reflection, under the hypothesis that the scenario in question obtains. Analysis of a concept proceeds at least in part through consideration of a concept's extension within hypothetical scenarios, and noting regularities that emerge" (CARE: 7)

"This pattern, whereby a conditional ability to evaluate a concept's extension yields elucidation of a concept without a finite counterexample-free analysis, is illustrated very clearly in the case of 'knowledge' " (CARE:8)

"The possibility of this sort of analysis is grounded in the following general feature of our concepts. If a subject possesses a concept and has unimpaired rational processes, then sufficient empirical information about the actual world puts a subject in a position to identify the concept's extension" (CARE: 8)

"If something like this is right, then possession of a concept such as 'knowledge' or 'water' bestows a conditional ability to identify the concept's extension under a hypothetical epistemic possibility, given sufficient information about that epistemic possibility and sufficient reasoning" (CARE: 9)

"But it remains the case that when a subject possesses a name, the subject will have a conditional ability to identify its extension given sufficient empirical information about the actual world, and the relevant conditionals will be a priori for the subject" (CARE: 12)

"It is probably easier, then, to give up the aim of producing a perfect explicit analysis, and to content ourselves with the observation that we have an a priori grasp of how our concepts apply to specific epistemic possibilities, when these are described in sufficient detail" (CARE: 24)

"The epistemic intension is a function, not a description. It is revealed in a subject's rational evaluation of specific epistemic possibilities, not in any sort of explicit definition" (SI)

"Here the crucial property of a description is that it gives us a way of identifying an expression's extension, given full knowledge of how the world turns out. It may be that for some expressions ..., there is no description that can do this job. It is nevertheless not implausible that the expression's extension depends in some fashion on how the world turns out, and in particular that full knowledge of how the world turns out puts a subject in a position to identify the expression's extension" (SI)

As these passages make clear, a given expression need not be associated with a description in order for it to have an extension in a given (conceivable) scenario. But if the subject possesses the concept in question, then she has an ability to pick out the extension at any (conceivable) scenario. Here is another way to put the point: to grasp a concept simply is to have this ability (under ideal circumstances). If a subject lacks the ability (under ideal circumstances), then she does not master the concept.