Having posted on Mackie’s argument from relativity some time ago, I’d like to return to it now and ask whether the argument (or at least what I think is the best version of it) is inconsistent with other components of Mackie’s error theory.
In its simplest form, the argument claims that first-order moral disagreement is sufficiently deep and pervasive that the best explanation of this disagreement is that there are no objective moral facts to which we can appeal to settle such disagreement. In short, fundamental moral disagreement (disagreement at the level of basic moral principles or beliefs) is rationally intractable. If this is so, then moral disagreement cannot be traced to one party to the disagreement having the correct conception of the relevant moral facts, while the other has an incorrect conception.
We can put this in more exact terms:
A maintains that intentional killing of the innocent is always wrong.
B maintains that intentional killing of the innocent is not always wrong.
Since their disagreement only casts doubt on the existence of objective moral facts if it is rationally intractable, then let us equip A and B with all the rational tools necessary in order to isolate their disagreement and test its intractability:
Super-A is A but with perfect non-moral knowledge, no defects of rationality, etc.
Super-B is B but with perfect non-moral knowledge, no defects of rationality, etc.
Now, would super-A and super-B come to agreement — or is it possible that they would continue to disagree? We can, I think, grant that they might come to an agreement. Yet if we believe that it is possible for their disagreement to survive, then this is a powerful reason to conclude that there are no objective moral facts: Intuitively, since super-A and super-B have not settled upon what the moral facts are, and we cannot attribute this to a rational or epistemic shortcoming of super-A or of super-B, then the most economical explanation is that neither A nor B have failed to discern the moral facts correctly because there are no moral facts for them to correctly discern.
But notice that if this is correct, then the disagreement must originate in some non-rational state of super-A and super-B, perhaps (as Mackie suggests) to their commitments to competing ‘ways of life’. I gather that such commitments must, in order to honor the stipulations of the argument I just gave, be desire-like rather than belief-like: For if they were belief-like, then because belief can be rational or irrational, the commitments themselves would be candidates for what super-A and super-B are cognitively equipped with. This doesn’t entail that super-A and super-B do not have a disagreement in belief. (If it did, then Mackie would not be a cognitivist and an error theorist, but a non-cognitivist.) But it does strain the Humean psychology that Mackie seems to defend (notably in sections 6 and 9 of Ethics, chapter 1). If beliefs and desires are original existences such that the existence of a belief never implies the existence of a desire and vice versa, then we need an account of why an individual’s non-rational commitments become regularly correlated with certain moral beliefs. Why does A’s commitment to a way of life, one strongly opposed to killing the innocent, necessarily result in her *believing* that intentional killing of the innocent is always wrong? That it does is a coincidence in need of explanation.
I don’t know if this point has been discussed before (maybe by quasi-realists?), so if anyone’s able to point me in the right direction …