Jackson on Truth at Worlds Considered as Actual
Something interesting seems to happen on pp.261f. of Frank Jackson's "Why We Need A-Intensions" (Phil. Studies, March 2004):
How is truth at a world under the supposition that that world is the actual world related to truth at a world simpliciter? It would be good to have an assurance that there are no problems special to the former, as Ned Block convinced me [...]. For some sentences, their A-intension is one and the same as their C-intension. [...] For them, truth at a world and truth at a world under the supposition [that] it is the actual world are one and the same. There is a difference between a sentence's A- and C-intensions if and only if the evaluation of the sentence at a world requires reference back to the way the actual world is as a result of some explicit or implicit appearance of "actually", or an equivalent rigidification device, in the sentence. But when this happens, the role of worlds in settling truth values is the standard one, the one that applies when it is C-intensions that are in question. The only difference is that the value at every world but one depends in part or in whole on how things are at another world. There is no difference in the role of how things are at worlds in settling truth values; the difference is in which worlds are in play. To put the point in terms of a simple example: (a) "The actual F is G" is true at w under the supposition that w is the actual world iff "The F is G" is true at w; and (b) what follows "iff" in (a) contains "is true at w" and not "is true at w under the supposition that w is the actual world".
Many people have complained that they don't understand what it means to evaluate a sentence in a world considered as actual, or that however that is to be done, it won't deliver the results Jackson promises.
In the quoted paragraph, it looks like Jackson offers a new kind of explanation: Let S be a sentence token and w a centered world. We want to know whether w is in the A-intension of S. According to Jackson, we first have to find out whether S contains an explicit or implicit rigidification device. If not, w is in the A-intension of S iff it is in the C-intension of S. It is not so clear what happens when S does contain a rigidification device. The example suggests that in this case w is in the A-intension of S iff w is in the C-intension of some other sentence S', where S' is a de-rigidified version of S.
Note how different this is e.g. from both the contextual and epistemic accounts discussed in Chalmers's Foundations. There is no mention at all of an S token at w, nor are there any epistemic primitives such as apriority in Jackson's account.
I like all this a lot, though I'm not sure whether Jackson's explanation works, and how exactly it is to be understood. A minor worry is that there may be problems with indexicals and tenses in the sentence S: Do we have to translate a given token of S into an non-indexical, eternal, de-rigidified sentence S' before we can figure out whether w is in its A-intension? (If not, the C-intension of S' will itself be context-dependent, but what is the relevant context: the context in which S was uttered? won't that create problems e.g. with token-reflexives?) A more serious problem is that de-rigidification may not be possible in our language: Assume "water" is rigid, but there is no (non-rigid) description D in English such that "water" is co-intensional with "the actual D". Then what are we to put in place of S'?
Perhaps I've misunderstood the example. Perhaps Jackson doesn't mean that we always proceed via de-rigidification. But then I'm not sure I understand his proposal. It at least sounds as if he suggests that there are general rules for the evaluation of a sentence at a world considered as actual that presuppose only knowledge of how to evaluate a sentence at a world considered as counterfactual.