Shelly Kagan on Moral DisagreementNormative Ethics (Westview Press: 1998), p. 9.
Of course, the study of the moral beliefs of different cultures can be helpful in a number of ways. It can open our eyes to the fact that different groups have disagreed about moral questions — even on some of the matters that seem most self-evident to us. If nothing else, this may deepen our desire to discover to what extent our own moral views can be defended. And it may leave us more open to the possibility of deciding that it is actually some of our own moral views that are mistaken and in need of revision. Furthermore, the study of the moral beliefs of other groups can help us discover arguments for or against some position — arguments that we might otherwise have overlooked but that are worthy of careful consideration. And, of course, the study of the moral beliefs of other groups can be interesting in its own right.
All these same things are true with regard to studying the theories of the great moral philosophers, living and dead. Learning about these philosophers can be fascinating in its own right and can drive home to us the realization that the problems of normative ethics, and of moral philosophy in general, are deep and difficult, and even thoughtful and brilliant people can disagree abou the answers, And — not surprisingly — studying the writings of moral philosophers is one of the best ways to discover rival moral theories, as well as some of the most interesting and compelling arguments for and against these theories.