This reply by Sam Harris is interesting, and I do thank him for his care and civility. I'm afraid that I find crucial aspects of it unconvincing, but I'm grateful that we can have a reasonable, good-humored dialogue on some of these issues. As I often do, I'll respond in more than one part, as there are many threads to try to untangle.
At this stage, let me just say that there is some agreement between us. I don't think that the process of formulating public policy, or the process of considering how we ought to regard another culture's customs and laws (or those of our own culture(s)), or the process of thinking about how we should, as individuals, live our respective lives, is just hopeless, or that the results of deliberation on these things are inevitably going to be merely arbitrary or idiosyncratic. On the contrary, there's a good chance that the results of serious deliberation will have taken account of all [edit: or at least many] of the relevant facts and then combined them with desires, values, etc., that are widely shared and can be understood and accepted by most people. These processes can be rational and reasonable ones.
Harris compares them to the practice of medicine, which can, I agree, also be a rational and reasonable one. If I were denying that the practices/processes mentioned in the previous paragraph were (or were capable of being) rational and reasonable, Harris would be able to use the practice of medicine as a compelling counter-example, suggesting that my standards are too demanding. But I don't deny any of that. My point at this stage of the argument is somewhat narrower. I doubt that there is a metric for well-being (and I believe that Harris has still not thought through the implications of this, even though he appears to agree). I also argue that the practice of using such a metric, even if it existed, would not be binding on us in such a way that we'd be required (perhaps as matter of rationality) to maximise the sum of well-being.
Sure, medicine or healthcare may be able to get by without a single, objective metric. I happily concede that. I also concede that the processes of critiqueing laws, customs, and so on, can do likewise. They are not thereby condemned to be irrational or unreasonable, or to be useless, or hopeless, or arbitrary. But, whatever anyone else is saying, that is not my position. My position is, rather, that we can make rational, reasonable, non-arbitrary, etc., judgments about the "goodness" of systems of law and custom, and so on, without needing those judgments to be objectively "correct". If anything, the analogy with medicine supports my position on this, rather than being a counter-example. It shows how a practice can operate rationally, reasonably, and so on, even without in all cases giving us objectively binding guidance on how we should act.
However, if someone thought that there really is an objectively binding prescription for action in all cases in the medical and healthcare area ... she would be mistaken. Further, it would be worthwhile pointing this out: it would be of philosophical interest and practical importance. If it were the commonsense view, philosophical scepticism about common sense in this domain would be defensible ... and perhaps even useful. Harris seems to get this, but doesn't draw the obvious conclusion, by analogy, that moral objectivism is neither necessary (for practical purposes) nor correct (at a theoretical level).
I'm only clearing my throat here - letting y'all know that I really am thinking about this. I'll get into some more detail tomorrow. I think the analogy with medicine/healthcare may be a useful place to start, so I'll begin tomorrow by elaborating on some of the above.