When I teach moral theory, I stress that the theories are supposed to be extensionally equivalent (that is, yield the same decisions as the others at least in cases considered clear)—even though they approach decisions in radically different ways. Any theory not extensionally equivalent to the others will, in that respect at least, be open to counter-example, and every counter-example makes a theory less appealing. The great moral theorists are great in part at least because they found ways for their moral theory to absorb (or otherwise disarm) many of the supposed counter-examples (and related objections). Moral theory is a sort of arms race between theorists who develop new counter-examples (or related objections) and theorists who find ways to absorb them into the theory. The theories as such, the few simple principles that constitute their opening statement, are not what make moral theory interesting to those of us who like doing moral theory. Those principles generally come from common sense, philosophers contributing little more than clarity and precision. What makes moral theory interesting is the arms race. Can we find a counter-example that will shake up those who defend this or that theory? Can we absorb this or that supposed counter-example that now seems to threaten our theory?
(Michael Davis, "The Usefulness of Moral Theory in Practical Ethics: A Question of Comparative Cost (A Response to Harris)," Teaching Ethics 10 [fall 2009]: 69-77, at 75-6 [italics in original])
Note from KBJ: I don't teach moral theory like this. Theories are not supposed to be extensionally equivalent; they're supposed to diverge. That's what makes them different theories! A normative ethical theory (such as theological voluntarism, utilitarianism, egoism, Kantianism, Rawlsian contractarianism, or Rossian pluralism) is a sorting device. It sorts acts into two categories: those that are permissible (i.e., right) and those that are impermissible (i.e., not right, or, simply, wrong). Different people will be attracted to different theories. I, for example, find that egoism best systematizes my beliefs and values. If a conflict arises between my theory and a particular judgment that I make (or am inclined to make), I must either revise the theory or abandon the judgment. The data of ethics are subjective, whereas the data of science are objective. The only way to argue in ethics is to show others that their own theory commits them to (1) accepting something they don't already accept or (2) rejecting something they do already accept. In other words, ethical argumentation is ad hominem (to the person) in the Lockean sense.
