How to Derive "Ought" from "Is"

Back when I was in college and taking up philosophy, the received opinion concerning ethics claims, the standard doctrine espoused by all my teachers, was that, since Hume at least, we can all agree that one can't derive "ought" statements from "is" statements, that is claims about what we ought to do in any given case do not follow based on the descriptions of the facts of the case alone. Of course, this is moderated somewhat by the realization that some "is" statements present us with reasons to make "ought" claims to the extent that we are so inclined and that we believe others share the same inclinations that we do. Confronted with a fact that prompts us to choose X, for instance, we will naturally expect that someone else with values like ours will be susceptible to the same prompt and recognize the same reason to act as we do. To the extent moral assertions are built on that, it is possible to move in a seemingly logical way from what there is to what we ought to do about it. But the problem, particularly in the moral case, boils down to situations where the prompts themselves are in question.

If seeing someone in danger or in pain serves to prompt me to try to alleviate the conditions causing the other person pain or putting them in danger, it doesn't follow that that prompt will have the same effect on someone else. Nor does it follow that it should have that effect on me if it so happens that it doesn't. This is the problem of deriving oughts from is's. And it lies at the very heart of the moral case.

Since Hume this has been standard stuff in moral philosophy . . .

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