Marc LiVecche writes (Does Can Imply Ought?):
I want to be clear that I do not believe just because prudence overrules obligation that somehow the obligation ceases to be one. Not quite. I’m keen to make some space between my claims and what I take to be the basic Kantian premise that ought implies can. The opposite of my eponymous question, Kant’s phrasing insists that if a particular action is impossible for an agent to perform then the agent is under no obligation to perform it. Against this, I want to suggest that the obligation still remains; it’s just that it cannot be met. In some cases, there may be no moral culpability attendant in my not being able to carry out the obligation. In other cases, there might well be. If the reason I could not perform the rescue is only because I was too blindly drunk to do so, I may be deemed morally blameworthy indeed.
The other argument against the proposition that can implies ought is that of personal—or national—interest. Again, the beach: I may be a great swimmer but if I’m alone with my young child on the otherwise deserted beach and the conditions are such that attempting rescue is mortally perilous to me, then not just prudence but parental responsibility might dictate I refuse to try. In this scenario, it should become clearer that it need not be a question of whether I have a duty to attempt the rescue or not. The duty to rescue the drowning man is now conflicting directly with my duty to not put my child at undue risk. I am not shirking responsibility, I am weighing one set of responsibilities against another set and, finding that it is impossible to meet both, I determine that I have grave reasons to give deference to the one set over the other. The accompanying sentiment here should be one of tragedy.
What I am trying to champion here is a modification of the so-called Spiderman ethic: with great power comes great responsibility. My thesis is that there is always a strong presumption that if there is someone in danger then it is obligatory that I do something about it, conditioned only on whether I have the capacity to. If I do not have the capacity, it does not mean I am simply off the hook. It means that in this regard I am a failure. This carries an implication: to the degree that we are reasonably able, we ought to increase our capacities. Of course, sobriety is called for. At a personal level, there are many ways to help those in need. But not everybody can be expected to cultivate all the financial, intellectual, physical, medical, martial, and other capabilities required to help our neighbors in any crisis they face. Obviously, too, our obligations are primarily to those special obligations unique to who we are: they radiate outward from our immediate family to our extended family, our friends, our neighbors, those to whom we have made promises, those encompassed in our sworn oaths, our countrymen, etc. In some situations our greatest duty might well be owed simply to the person immediately in front of us. But to the degree that we are able, within reason, to cultivate the capabilities of meeting possible obligations without compromising existing obligations we ought to.