Historiographical reflections

I know this "scumbag analytic philosopher" meme is played out, but this one just came to me:

Commence groaning...


(Why) must ethical theories be consistent with intuitions about possible cases?

Since Brian Weatherson recently classified this blog as 'active,' I thought I should try to live up to that billing.

A question came up in one of my classes yesterday. The student asked (in effect): Why wouldn't a moral theory that makes all the right 'predictions' about actual cases be good enough? Why demand that a moral theory must also be consistent with our intuitions about possible cases, even science-fiction-ish ones, as well?

(The immediate context was a discussion of common objections to utilitarianism, specifically, slavery and the utility monster. The student said, sensibly I think, that the utilitarian could reply that all actual cases of slavery are bad on utilitarian grounds, and there are no utility monsters.)

I know that some philosophers have argued that if a moral claim is true, then it is (metaphysically?) necessarily true: there is no possible world in which e.g. kicking kittens merely for fun is morally permissible. If you accept that all moral claims are like this, then I can see why you would demand our moral theories be consistent with our intuitions about all possible cases. But if one does not accept that all moral truths are metaphysically necessary, is there any other reason to demand the theory make the right prediction about merely possible cases?

This question seems especially pressing to me, if we think one of the main uses of moral theories is as a guide to action, since we only ever act in the actual world. However, now that I say that explicitly, I realize that whenever we make a decision, the option(s) we decided against are merely possible situations. So maybe that could explain why an ethical theory needs to cover merely possible cases? (Though even there, it need not cover all metaphysically possible cases -- e.g. the utility monster worries never need to be part of my actual decision-making process.)