I regularly appeal to the moral intuition that we have a strong obligation to leave strangers alone, but little obligation to help them:

What are you morally forbidden to do to a stranger?  You may not murder him.  You may not attack him.  You may not enslave him.  Neither may you rob him.

What are you morally required to do for
a stranger?  Not much.  Even if he seems hungry and asks you for food,
you’re probably within your rights to refuse.  If you’ve ever been in a
large city, you’ve refused to help the homeless on more than one
occasion.  And even if you think you broke your moral obligation to give, your moral obligation wasn’t strong enough to let the beggar justifiably mug you.

These common-sense ethics regarding strangers, ethics that almost
everyone admits, are unequivocally libertarian.  Yes, you have an
obligation to leave strangers alone, but charity is optional.

One common challenge to my position is Peter Singer’s Drowning Child example:

To challenge my students to think about the ethics of what we owe
to people in need, I ask them to imagine that their route to the
university takes them past a shallow pond. One morning, I say to them,
you notice a child has fallen in and appears to be drowning. To wade in
and pull the child out would be easy but it will mean that you get your
clothes wet and muddy, and by the time you go home and change you will
have missed your first class.

I then ask the students: do you have any obligation to rescue the
child? Unanimously, the students say they do. The importance of saving a
child so far outweighs the cost of getting one’s clothes muddy and
missing a class, that they refuse to consider it any kind of excuse for
not saving the child. Does it make a difference, I ask, that there are
other people walking past the pond who would equally be able to rescue
the child but are not doing so? No, the students reply, the fact that
others are not doing what they ought to do is no reason why I should not
do what I ought to do.

Once we are all clear about our obligations to rescue the
drowning child in front of us, I ask: would it make any difference if
the child were far away, in another country perhaps, but similarly in
danger of death, and equally within your means to save, at no great cost
– and absolutely no danger – to yourself? Virtually all agree that
distance and nationality make no moral difference to the situation. I
then point out that we are all in that situation of the person passing
the shallow pond: we can all save lives of people, both children and
adults, who would otherwise die, and we can do so at a very small cost
to us…

This is the kind of moral argument I respect.  Singer isn’t starting with utilitarian absolutism and deducing absurdities.  He’s starting with a straightforward example and deducing plausible conclusions.  Every ethicist should emulate his approach. 

Still, Singer hasn’t fundamentally changed my mind about obligations to strangers.  Here’s why.

1. Note that my original answer to the question, “What are you morally required to do for a stranger” was “not much” rather than “zero.”  My wording was deliberate because I expected critics to bring up the Drowning Child counter-example.   But note how extreme an example Singer has to construct.  Mere “hungry beggar” isn’t good enough, as I initially pointed out.

2. Singer asks his students whether or not they have a “moral obligation” to save the Drowning Child.  But he could just as easily have asked: “Is it morally praiseworthy to save the Drowning Child?”  I think almost everyone would agree that it is.  Indeed, we might even call the rescuer a “hero.”  My question: If the rescuer is merely fulfilling his moral obligation, why would anyone consider his action morally praiseworthy, or even heroic?  The right lesson to draw, I suspect, is that rescuing a Drowning Child goes at least slightly above and beyond moral obligation.

3. Suppose I’m wrong on point 2.  Singer also neglects another crucial distinction: an action can be morally obligatory without being morally enforceable.  Would it be morally permissible to point a gun at a person if he fails to rescue the Drowning Child voluntarily?  Much less clear.  To pull the trigger?  Even less so.

4. Singer is quick to move from the moral obligation to rescue a Drowning Child to a moral obligation to rescue lots of strangers.  But suppose we revise his initial hypothetical: Instead of one Drowning Child on a random day, there’s a new Drowning Child every day.  Indeed, suppose there are Drowning Children as far as the eye can see, 24/7.  What then?  Now it seems clear that a policy of rescuing strangers is above and beyond the call of duty.  And sadly, as Singer himself points out, this is the world we live in.

Last point: Singer is quick to conclude that we have massive obligations to help millions of desperate strangers.  But all First World countries violate a far less controversial obligation to the world’s poor.  Under the guise of “immigration policy,” every First World country makes it a crime for foreigners to accept a job offer from a willing First World employer.  Economically speaking, this is a big deal: The best evidence says that worldwide free trade in labor would double world GDP and vastly increase Third World wages. 

We shouldn’t talk about the world’s poor like they’re Drowning Children.  If we lived up to our basic obligation to leave strangers alone, the world’s poor wouldn’t need our charity.