There are reasons to doubt that the so-called naturalistic fallacy is really a fallacy, but at this point I want to accept the major points of Hume's version of the open question argument and see how that actually helps us in comparing metaethical theories. The idea is that even if everybody agrees on the facts regarding our evolutionary history we might still disagree about which features of that story convey or produce moral value. We can use this difference to distinguish moral theories that otherwise tell the same story. It also lays bare those aspects of a evolutionary history that might do the metaethical job and thus reveal whether any such story will do the job.

The same problem has three names (naturalistic fallacy, is/ought gap, open question argument), and though some have drawn nuanced differences among them, we'll take them as the same issue. The idea is that given any naturalistic explanation of a moral phenomenon in terms of a collection of facts about the world (i.e. "is" statements) it is always an open question whether any particular moral claim (i.e. "ought" statement) is true. Hume argues rather forcefully that for the conclusion of any argument to be an evaluative claim one of the premises must be an evaluative claim. My rather simplistic but useful invocation of this idea is that in giving our evolutionary story for the existence of morality we have to identify which feature of the story is the one that produces morality.

There is no doubt that a capacity for morality, if not moral value itself, is woven into the evolutionary past of humans. Humans experience a unique emotional phenomenon that we've called "moral" and it's rather unclear how our environment prompts these specific feelings and what reproductive benefit they could have provided to our ancestors. The biological account of humans' evolution need not invoke morality at all; natural selection via our physical environment (including conspecifics interaction) suffices. Then it seems we just have to choose some feature of the human experience (maybe something like the survival of the species itself) and make that the yardstick for morality. And from there we could see how that being the morally producing feature would provide reasons for or against certain actions, generate motivation or not, etc.

The approach I'd like to take is to respond to the open question argument by admitting that thinking of some behavior as moral is different from thinking of some feature or process of our evolutionary past, but what makes the behavior moral (or otherwise) is that it played the role that it did in our evolutionary history. This is different from a scientific explanation of why humans feel certain behaviors are moral or otherwise; that explanation allows room for mistakes in moral judgment and seems to be the focus of the moral psychology research. The mistakes happen because whatever really is morally correct may not have been the thing driving the evolution of our moral psychology and so we likely feel differently than we ought to. My account differs because I am ignoring moral psychology and trying to identify features of our evolutionary past that may properly and sensibly be the thing that, if taken as the giver of goodness, produces a plausible moral theory with at least some benefits of some existing accounts. More important that the resulting metatheory will be the arguments that the only way to approach this problem is to focus on behavior rather than psychology, and that is both the major departure and primary insight of the work.