According to Christine Korsgaard, there is a distinction between explanatory and normative adequacy for a moral theory. In a really interesting passage, Korsgaard asks us to think from two points of view - a third person point of view and a first-person point of view. She then takes a standard evolutionary explanation of moral obligation and asks us to consider its truth from both points of view:
Suppose someone proposes a moral theory which gives morality a genetic basis. Let's call this 'the evolutionary theory'. According to the evolutionary theory, right actions are those which promote the preservation of the species, and wrong actions are those which are detrimental to that goal. Furthermore, the evolutionary theorist can prove, with empirical evidence, that because this is so, human beings have evolved deep and powerful instincts in favour of doing what is right and avoiding what is wrong. Now this theory, if it could be proved, would give an account of our moral motives which was adequate from the point of view of explanation. Our moral instincts would have the same basis and so the same kind of power as the sexual drive and the urge to care for and defend our children. And we know from experience that those instincts can induce people to do pretty much anything, even things which are profoundly detrimental to their own private interests or happiness.
But now ask yourself whether, if you believed this theory, it would be adequate from your own point of view. Suppose morality demands that you yourself make a serious sacrifice like giving up your life, or hurting someone that you love. Is it really enough for you to think that this action promotes the preservation of the species? You might find yourself thinking thoughts like these: why after all should the preservation of the species count so much more than the happiness of the individuals in it? Why should it matter so much more than my happiness and the happiness of those I care most about? Maybe it's not worth it. Or suppose the case is like this: there are Jews in your house and Nazis at the door. You know you will get into serious trouble, even risk death yourself, if you conceal the Jews. Yet you feel morally obligated to risk death rather than disclose the presence of the Jews. But now you know that this motive has its basis in an instinct designed to preserve the species. Then you might think: why should I risk death in order to help preserve the species that produced the Nazis? (Sources of Normativity, 14-15)
The upshot of Korsgaard's reflection is that while evolutionary theory might explain an action, it "would not justify it from your own point of view." (15). Thus, though a theory might offer an explanation for our motivations, it may not be normatively adequate. This is because:
The normative question is a first-person question that arises for the moral agent who must actually do what morality says. When you want to know what a philosopher's theory of normativity is, you must place yourself in the position of an agent on whom morality is making a difficult claim. You then ask the philosopher: must I really do this? Why must I do it? And his answer is his answer to the normative question.To achieve normative adequacy for a moral theory one must explain both how we might come to have moral instincts and what makes those instincts sufficiently motivating for an agent to perform an action. This is because, obligatory actions are strongly motivated actions. We are strongly motivated to praise good actions and condemn bad actions. We aspire to emulate the life of the virtuous and eschew the life of the wicked. We are incensed by cases in which an evil-doer faces no punishment or a hero is forgotten. On occasion, our motivations lead us to perform actions for which there is significant personal cost. The point is: we sometimes perform those actions solely for a moral reason.
So, what exactly is it that motivates us?
Robert Adams argues that obligations are necessarily social in nature: If there is an obligation, then there is a social context. He argues that when we fail to perform an obligatory action, we experience guilt. The experience of guilt implies that we have (i) harmed another person and (ii) alienated ourselves from other people.
The problem with such an explanation is that it seems the obligations are only in place as long as there is a social group in which those obligations are binding. If obligations are identical to actual demands made by social groups, then there are only non-objective social obligations. This is so for two reasons. First, social groups might not exist. And if they didn't obligations wouldn't either. But could we really believe that murder, rape and a host of evil actions would be morally justified? It may be the case that no one could carry out any of these actions, but it is implausible to suggest that their status changes depending on the existence of social groups. Second, social groups might demand different obligations. One social group might decide against imposing an obligation to tell the truth on its members.
What would be lost in both cases is the objective nature of obligations. As Korsgaard suggests, it is the objective nature of obligations that seems to propel people to act to meet them even at great personal cost. It is far more plausible that there are objective social obligations. And, if so, then obligations are not identical to actual demands made by social groups.
Consequently, Adams goes on to argue that if obligations are social in nature, then God exists: If obligations are placed upon us by God, then those obligations are both social and objective. They are permanently in place and thus sufficiently motivating. This seems eminently plausible. God's existence would explain the force of obligations in a way not open to the naturalist.