I have long subscribed to Kant's famous meta-ethical principle according to which our moral obligations cannot outrun our abilities. 'Ought' implies 'can.' If I am under a moral obligation to do X, then I must be able to do X. We are concerned here with moral not legal oughts, and we understand 'ought' in accordance with the principle that if one morally ought to do X, then one is morally obliged/obligated to do X.
Roughly, if you ought to do something, then it must be possible for you to do it, not just logically, and not just nomologically; it must be possible for you to do it given your actual abilities at a particular time and in definite circumstances. With a bit more precision:
OC. Necessarily, if agent A ought to do X at time t in circumstances C, then A is able to do X at t and in C.
So if I ought to come to your aid, then I am able to do so. By contraposition, if I am unable to come to your aid, then it is not the case that I ought to, and I am not subject to moral censure if I fail to.
Note the logical difference between 'It is not the case that A ought to do X' and 'A ought not to do X.' To confuse those two would be to commit an operator shift fallacy by importing the negation operator into the negatum. So the contrapositive of 'ought' implies 'can' is not 'cannot' implies 'ought not,' but 'cannot' implies 'not ought.' Better still: 'not can' implies 'not ought.'
Now suppose I promise to drive you to the airport at six in the morning. So promising, I morally obligate myself to so doing, i.e., I ought to drive you to the airport at six. It follows by (OC) that I can drive you to the airport in a very concrete sense of 'can,': I know how to drive; I know how to get to the airport; I have access to a car, no one is preventing me from driving, etc. Obviously, a carjacking would absolve me of my moral obligation.
My ability in this concrete and specific sense is a necessary condition of my being morally obligated to drive you to the airport.
Putative Counterexample
Suppose that the night before the airport run I get drunk, sleep through the alarm, wake up late and hungover, and forget to fill up the gas tank in my vehicle. As a result we run out of gas and you miss your flight. I am unable to deliver on my promise, and do what the promise obligated me to do, but it seems that I am nonetheless morally responsible and indeed open to moral censure. In this case it seems that 'not can'' does not imply 'not ought.' It seems that my inability to get you to the airport on time does not absolve me of my moral obligation to perform than very action. For I did something blameworthy by getting drunk the night before.
I am not impressed by counterexamples of this sort. Touching only the letter, but not the spirit of Kant's great principle, they merely invite a reformulation thereof. To wit,
OC*. Necessarily, if agent A ought to do X at time t in circumstances C, then A is able to do X at t and in C subject to the proviso that around t and in C A has not done anything to impair his abilities or factors contributing to his abilities.
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