It's worth noting this blurb from the Stanford Encyclopedia of Philosophy (available here):
The Argument from Relativity (often more perspicaciously referred to as “the Argument from Disagreement”) begins with an empirical observation: that there is an enormous amount of variation in moral views, and that moral disagreements are often characterized by an unusual degree of intractability. Mackie argues that the best explanation of these phenomena is that moral judgments “reflect adherence to and participation in different ways of life” (1977: 36). This, at least, is a better explanation than the hypothesis that there is a realm of objective moral facts to which some cultures have inferior epistemic access than others. The example Mackie uses is of two cultures' divergent moral views regarding monogamy. Is it really plausible, he asks, that one culture enjoys access to the moral facts regarding marital arrangements whereas the other lacks that access? Isn't it much more likely that monogamy happened to develop in one culture but not in the other (for whatever cultural or anthropological reasons), and that the respective moral views emerged as a result?
Opposition to the Argument from Relativity can, broadly speaking, take two forms. First, one might deny the empirical premise, arguing that moral disagreement is not really as widespread as it is often made out to be, or at least arguing that much of the conspicuous disagreement masks extensive moral agreement at a deeper level (a level pertaining to more fundamental moral principles). Mackie makes some brief remarks in response to this argument (1977: 37). Second, one might accept the phenomenon of moral disagreement at face value but deny that the best explanation of this favors the error theory. Often both strategies are deployed side by side.
Aaron and Chelsea characterize Mackie's argument in slightly different ways and theirs are different from the way that it's put in the SEP. It will thus be helpful for our discussion to consider more formal/precise statements of Mackie's argument. Following the SEP entry, here's one way to put the argument:
Mackie's Argument from Relativity (or, better, Argument from Disagreement)
(1) There is genuine (and seemingly intractable) moral disagreement. (This is an empirical observation that Mackie made.)
(2) One explanation for this disagreement is that there are objective moral truths/facts and some cultures have inferior epistemic access to these facts than others, i.e., some cultures are better when it comes to recognizing the moral facts than others. On this view, some cultures are correct in making certain moral claims while others are incorrect.
(3) Another explanation of moral disagreement is that there are no objective moral truths, and that moral disagreement is simply a result of cultures developing differently. Some cultures developed in such a way that monogamy is deemed morally good and others didn't develop in this way. On this view, there is no "right" answer when it comes to a moral issue--neither party in a moral dispute is correct or incorrect.
(4) The explanation described in (3) is a better explanation of moral disagreement than the one described in (2). This is because it's simpler (not as many entities need to be credenced, etc.) and it involves a more plausible causal story about why there is moral disagreement (i.e., it's more plausible to think that anthropological/sociological facts lead to moral disagreement than that disagreement is due to different cultures having varying degrees of epistemic access to the moral facts that exist).
(5) Therefore, we ought to think that the explanation in (3) is the correct explanation. So, we should think that there are no objective moral facts/truths.
(6) If there are no objective moral truths, then no moral claims are true.
This might not be the best way to characterize Mackie's argument. Feel free to comment on this post if you think there's a more accurate way to put it. In addition, independent from considerations about textual accuracy/being sure that we're characterizing Mackie's argument correctly, is there a more powerful way to put the argument? In other words, is there a more powerful way to show that error theory ought to be accepted from the premise that moral disagreements exists?
And let's not forget Aaron and Chelsea's posts. They didn't put Mackie's argument in premise-conclusion form. How might we more formally phrase the argument as they characterize it? In addition, how might their objections be applied to the formulation of Mackie's argument above? Would they claim that the explanation in (2) is actually to be preferred over the one in (3)? What justification might one have for this?
It will be easier if all comments related to Mackie's argument are made to this post.
And thanks to Aaron and Chelsea for volunteering to go first as guest bloggers.