Moral error theory interprets moral judgments as making truth-apt claims - and says they are all false. Now, in fact, it may not be so simple. While that's a position within a set of metaethical pigeonholes, no one who sees a problem with moral judgments necessarily has to claim that all moral judgments are simply false. Morality may turn out to be very messy. Perhaps some moral claims are best interpreted non-cognitively, and so as neither true nor false. Perhaps some are best interpreted cognitively, or as having a cognitive component, but are actually true. In The Myth of Morality, Richard Joyce thinks that moral language is pretty much seamless, so that all sorts of first-order moral judgments stand or fall together. Here, I'm not sure that I'm with him - and this an area where I'd like to find some time for more thought and research.
Still, skipping over that for now, Joyce and I (and Mackie and Garner, and a lot of other philosophers these days) agree that what are often thought of as the most central moral judgments are literally false. Those are claims such as "X-ing is morally wrong" or "Y-ing is morally obligatory". Actually, even here I'm not sure that I'm in full agreement. I think that part of the trouble is that we don't really know what we're saying when we make these claims. This sort of moral language may suffer from confusion and uncertainty as much as anything else.
So, I'm starting to look like an error theorist in only a minimal sense. Nonetheless, one of the things that people seem to be thinking and conveying when they use this kind of language is "X-ing is objectively forbidden" or "Y-ing is objectively demanded". I.e., there's a claim of objective prescriptivity here, and my position is that all such claims are false. Sophisticated moral relativists also hold that all such claims are false, but they argue that we don't make such claims when we make moral judgments. Such positions - which are rather remote from the kind of crude moral relativism that Sam Harris rightly atttacks, and which almost all philosophers hate - make moral judgments seem much like ordinary evaluations. Or, in one version, moral judgments can be interpreted (perhaps charitably) as being like that.
Although moral relativism has a bad reputation, I think the most sophisticated versions, which tend to be inspired by the work of Gilbert Harman, are actually quite attractive. As I've observed before, what separates them from the most common form of error theory is their moral semantic component. Whereas as an error theorist will interpret "X-ing is morally wrong" as "X-ing is objectively forbidden" or maybe "X-ing is forbidden by a standard that is objectively binding", a sophisticated relativist will interpret it as "X-ing is forbidden by standards that those in involved in this conversation share" or perhaps "X-ing is forbidden by standards that I invite others involved in this conversation to share". There may be a further thought that the standards being used could be justified to others involved in the conversation, perhaps by appealing to their values and/or widely shared values.
The problem for sophisticated relativists is that we really do tend - don't we? - to think, when we make these kinds of thin judgments, that we are stating objective requirements or prohibitions. In some sense, the requirement or prohibition exists in the nature of things, or is just a fact (of some sort), or is true as a matter of reason (in some sense of "reason" that we cannot ignore). In that sense, these sorts of moral judgments are not like other evaluations of "good" and "bad" where we don't seem to think that the standards used are binding. As I've discussed in the past, other evaluations leave room for at least some legitimate disagreement. Perhaps more importantly, they don't usually involve the idea that the standard used is objectively binding, even on rational creatures who don't share the desires, goals, purposes, or whatever that underwrite it. We get by making evaluations that are useful to us in particular social contexts.
Towards the end of my interview with Common Sense Atheism the other night, I was asked a question to the effect of how I could reassure someone who is worried about the idea that the standards on which moral judgments are made are not objectively binding.
I had a fair bit to say, but I was conscious as I thought about it once more, that a lot of people really are going to find the idea disconcerting. What can be said to them, at the end of the day, is going to involve a whole lot of theory that they need to grasp, and something important does at least seem to be lost if moral requirements and prohibitions are not, in the sense that I'm denying, objective ones. Putting myself into the mind of an objectivist who is an ordinary good person (note again that I don't think that the word "good" is necessarily tainted with any error), it is quite a lot to be asked to give up the idea of objective prescriptivity.
As Richard Garner emphasises in his publications, there is also a dark side to objective prescriptivity. In the end, we may be better off without it. I'm definitely not suggesting that abandoning it will cause chaos and suffering. But it's a psychological wrench for most people, or at least that's my perception. I don't think we can simply embrace sophisticated relativist moral semantics, because it looks to me at the moment that the folk (and hence the language they use) are committed at least to some extent to the idea of objective prescriptivity. But nor can we expect them to give up that idea lightly. Sophisticated relativists like Harman make some good points, but they are too optimistic about our ability to expunge objective prescriptivity from our thinking and our discourse.
Again, the problem is not that chaos would ensue if we all agreed to think of moral judgments as much like other evaluations, finding an eventual foundation in various human values and projects - with the addition that these will be especially important and salient values, such as social survival and amelioration of suffering. I expect that we can get by just fine if we think like that, and it's a useful way to think when actually doing practical philosophy. I'll go on arguing this. The problem is the psychological wrench involved if we ask people in general, not just a few philosophers and philosophically-minded folks, to think this way.