R. M. Hare (1919-2002) on Obligation and Ability
The sense of ‘imply’ in which ‘ought’ implies ‘can’ is not that of logical entailment. It is a weaker relation, analogous to that which Mr. [P. F.] Strawson has claimed to exist between the statement that the King of France is wise, and the statement that there is a King of France. If there is no King of France, then the question whether the King of France is wise does not arise. And so, by saying that the King of France is wise, we give our hearers to understand that we think, at least, that the question arises to which this is one possible answer, and that, accordingly, there is a King of France. And similarly, if we say that somebody ought to do a certain thing, and ‘ought’ has its full (i.e. universally prescriptive) force, then we give our hearers to understand that we think that the question arises to which this is a possible answer, which it would not, unless the person in question were able to do the acts referred to.
(R. M. Hare, Freedom and Reason [Oxford: Clarendon Press, 1963], 53-4 [footnote omitted])