Richard Carrier over at his blog:
Moral realism is the view that there are moral statements that are meaningful and true, and true independent of your opinion or culture. I am a moral realist. That means I must be able to ontologically ground the existence of moral facts, and in things other than popular opinions or merely cultural facts. When I say they “exist” I have to explain what I mean by that: in what sense, and in what way, do they “exist,” particularly as I am a first-order physicalist (I believe everything that exists is solely and entirely caused by physical things and events: see Defining the Supernatural), so I must be able to reduce moral facts to physical facts in some way.
To get from A to B on this I have to drive by several destinations in the middle. Take, for instance, the scariness of an enraged bear: a bear is scary to a person (because of the horrible harm it can do) but not scary to Superman, even though it's the very same bear, and thus none of its intrinsic properties have changed. Thus the bear's scariness is relative, but still real. It is not a product of anyone's opinions, it is not a cultural construct, but a physical fact about bears and people. Thus the scariness of an enraged bear is not a property of the bear alone but a property of the entire bear-person system. And it is a physical property (it reduces entirely to the physical facts about bears and people and what the one can physically do to the other). Thus physical systems can have properties that their parts alone do not, yet that are entirely reducible to those parts and their physical arrangement (see Sense and Goodness without God, III.5.4.3 and III.5.5, pp. 128-34).
This scariness is also not simply subjective. Our emotional experience of fear is subjective, but the ability of the bear to harm us is an objective fact of the world. We can say “the bear does not scare me” if, for example, we don’t feel fear. But our emotion would then be misinforming us about the physical fact that the bear can seriously harm us (and therefore, in that sense, is objectively scary), regardless of what we feel. It would then be right to say “the bear ought to scare me,” and therefore the bear actually is scary and in this case we just don’t recognize this fact (we are, in other words, ignorant of the physical facts) or we recognize it (and thus acknowledge the bear is indeed scary) but do not feel the physiological indicators that usually attend that recognition.
Sean Carroll responds:
Carrier goes to great lengths to explain that these moral facts are not simply “out there” in the same sense that the laws of physics arguably are, but rather that they express relationships between the desires of particular humans and external reality. (The useful analogy is: “bears are scary” is a true fact if you are talking about you or me, but not if you are talking about Superman.)
I don’t buy it. Not to be tiresome, but I have to keep insisting that you can’t squeeze blood from a turnip. You can’t use logic to derive moral commandments solely from facts about the world, even if those facts include human desires. Of course, you can derive moral commandments if you sneak in some moral premise; all I’m trying to say here is that we should be upfront about what those moral premises are, and not try to hide them underneath a pile of unobjectionable-sounding statements.