MINERS: 10 miners are trapped in a flooding mine; they are either all in shaft A or all in shaft B. Given our information, each location is equally likely. We have just enough sandbags to block one shaft, saving all the miners, if they are in the blocked shaft, but killing them all if they are in the other. If we do nothing, the water will distribute between the two shafts, killing only the one miner positioned lowest. On the basis of these considerations, (1) seems true:
(1) We ought to block neither shaft.
While deliberating, though, we accept both
(2) If the miners are in A, we ought to block A
(3) If the miners are in B, we ought to block B.
We also accept
(4) Either the miners are in A or they are in B.
And (2)-(4) seems to entail
(5) Either we ought to block A or we ought to block B.
In a forthcoming paper, (“’If’s and ‘Ought’s,” JPhil), Niko Kolodny and John MacFarlane argue that the best way to resolve this paradox is to give up modus ponens. Instead, I’ll argue, acceptance of the contextualist semantics for modal expressions I advocate (in “A Flexibly Contextualist Account of Epistemic Modals” and “A Flexible, Contextualist Account of ‘Ought’”, http://www.unl.edu/philosop/people/faculty/dowell/dowell.shtml), together with a Kratzer-style semantics for the indicative conditional, allows for a resolution of the paradox without giving up on MP. This seems to me a clear advantage.