Over on the Center for Inquiry site, Ron Lindsay asks this question, which has also been occupying us here of late. Sample:
There have been innumerable articles, manifestos, and pamphlets that set forth some set of humanist values and principles that we are supposed to embrace. I don’t necessarily have a problem with the content of these lists of values and principles, but I am concerned that usually there is no explanation why a humanist or anyone else should adopt these values and principles. In other words, there’s little attempt to provide a method for approaching ethical issues. Sure, there is often a reference to using our reason, but although use of our reasoning powers is a good thing, by itself it doesn’t get you very far. If we are to be serious about developing a humanist morality, it seems to me that it is incumbent upon us to explain why we believe people should adhere to the values and principles that we advocate.
I am not proposing that we aim to develop a decision procedure that will generate the one right answer to all our ethical issues. Such a dreamy goal cannot be achieved, in part because there are a considerable number of ethical disputes to which there may not be one right answer. But we can develop a process of analysis and reflection that will provide some moral guidance by limiting the range of ethically acceptable answers -- which by itself would be a significant achievement.
One important element of our methodology should be a specification of the objectives of morality. If we can reach consensus on what morality is for , then it becomes a bit easier to resolve ethical dilemmas. One can critically examine a proposed course of action by considering whether it would further the objectives of morality.
Sam Harris would answer this question by saying that the purpose is "the well-being of conscious creatures", but that answer isn't obvious to everyone. (I've said before that I find it acceptable as an approximation, but even if that's right - and it's open to challenge - there's huge room to argue about "approximation to what?".)
Part of the problem is that it may be almost as difficult to get agreement on the answer to this question as it is to get answers to direct questions about which morality is "true". The purpose of something deliberately designed, like a car or a hammer, is pretty obvious in most contexts. When we're talking about something like morality, even the word "purpose" isn't quite right. Still, I think it's a good question, and you'll see me worrying at it further as time goes on. To me, it's one of the central questions of philosophy, as well as having practical urgency.