I've been thinking on and off for a long time (and without making too much progress) about counterfactuals with impossible antecedents. I have generally tended towards the view that they are trivially true: there are no possible worlds where P, hence every world where P is a world where Q. But this looks troubling when we want to distinguish between pairs like:
1. If the square root of 2 were rational, it could be expressed as n/m, with n, m integers.
2. If the square root of 2 were rational, it couldn't be so expressed.
Intuitively, you might think, 1 looks true and 2 looks false. Why, if they are both trivially true?
The answer that tempts me is that counterfactuals (including counterpossibles) are often used for, and often naturally heard as, expressing claims which are distinct from their literal contents. When we evaluate 1 as true and 2 as false, we are actually evaluating the sort of claim which is often conveyed by sentences like 1 and 2, although it is not what they literally mean. (In the case of 1, it might be something like the claim that any number which is rational can be written as n/m with n, m integers, so in particular this is true of the square root of 2).
A worry that a colleague put to me this morning concerning this view is that we have an intuition that it is wrong even to think things like 2, but stories which focus on the pragmatic communication in conversation of claims distinct from literal content will not capture that intuition.
I wonder whether one might respond that accepting in thought that 2 is true could be a mistake if it relied upon (or amounted to) mistakenly assuming that the non-literal content often conveyed by claims like 2 was true. If one accepted 2 in thought without realizing that it was only true because its antecedent was impossible, one might (e.g.) be making the mistaken assumption that no number which is rational can be expressed as n/m with n, m integers, so in particular this is true of the square root of 2.
It wouldn't be misleading to think 2 was true if one's reason for doing so was that one knew its antecedent to be impossible. But in this case I don't get the intuition that there is anything wrong or mistaken about thinking 2 is true.