Proximity shouldn’t make a difference — should it? We were on a beach on the European side of the Mediterranean, it was a beautiful late August day, and I felt so happy. The sea was fresh, the sky was clear and a stiff breeze was whipping white horses out past the headland.

‘Rough weather for migrants,’ I thought, then checked myself. What an awful thing to think. There really would be migrants out there somewhere over the horizon; desperate people; people who, had they been visible to me — were they, Heaven forbid, to start coming ashore — would have turned my perfect day into a day of torment.

But I couldn’t see them, and my perfect day continued unspoiled but for the slightest tickle of disquiet. Do I not have a right be happy, though others are drowning and I might be able to help them? Surely I do. They aren’t drowning here. I cannot see them.

Yet this cannot be so. Can my inability actually to see any desperate migrants throw some internal switch from Personal Moral Crisis to All’s Well? Or, to put it more analytically, how does an individual’s moral responsibility vary according to distance: distance not just geographically but in terms of social, family or national obligation?

We know that the answer lies somewhere between ‘completely’ and ‘not at all’ but cannot be located at either of those two extremes. You can rule out ‘no variation at all’ because it is simply impossible that you or I could feel toward every migrant on the shores or waves of the Mediterranean the same obligation we hold towards members of our close family. We could not function as moral beings if we felt equally beholden to all; as if every tolling bell really did toll for me.

But you can rule out ‘varies completely’, too, because our obligations towards those migrants cannot be set at zero. We wouldn’t wince at the television pictures if that were so. If those people’s fate was of no consequence at all to us, the coldly rational course would be to throw all of them into the sea, thus providing a disincentive for others to follow, and so end the nuisance. You or I would not advocate that, and those who pretend to believe it are simply posturing. All civilised humans, even the flintiest, believe there is some residual obligation to complete strangers.

So because the moral reach radiates outward in concentric circles from self at the core, to family, to friends, to community, to nation, to all humanity at the perimeter, we can agree that the command each successive ring can make upon our sense of personal duty weakens with distance from the centre but never entirely disappears. Scope for disagreement lies in how fast we may think it weakens, and where the critical mileposts are placed. There is room for disagreement, too, on the role played by chance: how far do we believe (as to some degree we all believe) that what fate throws in our path — the injured Samaritan you encounter by the wayside — can arouse an obligation?

I think I’ve fairly set out the terrain. But here’s the remarkable thing. Our Christian religion gives us — those of us who are Christians — almost no help in navigating it.

I say ‘our’ Christian religion because, though an atheist, I accept that my morality — and to a considerable degree what we call western morality — is anchored in a Christian culture. Whether or not we ourselves believe in God, we’ve all soaked up the ethical teachings.

And they’re useless. ‘Do unto others as you would that they would do unto you’ is, like ‘Love your neighbour as yourself’, either impossible or circular. Circular if it means ‘Act towards others as you think they should act towards you,’ because this leaves open the question of how you think people should act, and would permit a cannibal to eat another cannibal. If, however, the commandment means ‘Treat others as it would please you to be treated by them,’ then the precept is impossible because you’d give all your money to the first beggar you encountered. We’d all be off to Calais to try to smuggle migrants in our car boots.

Christianity (as represented by the Gospels) is all but silent — extraordinarily so — on the question of familial obligation. There is some evidence that Christ was impatient with family, and none that these were ties He wanted to reinforce. As to ‘community’, the parable of the good Samaritan suggests a certain impatience here, too. As to nation, I find ‘Render unto Caesar…’ an evasive answer, because it does not assist our understanding of which things ‘those things that are Caesar’s’ might be.

If, as I believe, the main difficulty that faces us in deciding moral duty is the difficulty of prioritising, then Christianity is profoundly unhelpful.

As an undergraduate just starting out on my chosen subject of moral sciences, I remember being shocked by a lecturer (I hate to speculate that it was Bernard Williams but I think it was) who told us airily that agonising ethical dilemmas were the stuff of philosophy lectures, but did not occur much in ordinary life. I thought and think that absolute rubbish. Literally millions of people across Britain have, over the past few months, been torn and distressed by the question of what ‘we’ should do about the migrants crossing the Mediterranean. I believe this has quite seriously spoiled peace of mind for many. Going further, I’d say this does not arise from baulking at a duty we know we owe, but from deep and genuine doubt as to what our duty is.

I’ll go even further. If I really knew my duty I would try diligently to do it; and there are millions like me. We’re ready. But we’re honestly, mightily, daily confused about what we ought to do.