The default option is acting as though others are of value, given our other beliefs.
If we don’t act as though others are of value, given our other beliefs, then we are committed to believing that we ourselves are both of value and not of value.
In that case, we can act only in ways that consistent with both. (As already pointed out, in the sixth paragraph of “The Origin of Morality,” this is possible. Drinking coffee is compatible with “Socrates is mortal.” It is also compatible with “Socrates is immortal.”) But if we can only act consistently with both, then there is no advantage to believing that we are of value. So, we treat others as being of value in order to keep the advantages of believing that we are of value.
Instead of doing the right thing, we can deceive ourselves into thinking that we are acting consistently by describing our actions to ourselves inaccurately. Thus, there is not just ignorance and error; there is also motivated ignorance and error. Self-deception does not change the moral facts, as already noted. Moral facts are constituted by the relationship between actions and the belief that we are of value. Believing falsely about the nature of actions does not change the true nature of the actions. Rape is rape even if the rapist falsely believes that his victim consented.
Self-deception does not always succeed, primarily because it is in the interests of others to keep us believing truly and we cannot get away with it but also because it can be difficult to come up with a plausible rationalization in some cases.
We vary in our ability to deceive ourselves. People without much ability to deceive themselves are the people who become moral saints and heroes.
Thus, by being rational and seeking the truth, we not only satisfy a precondition for acting morally but also increase the probability that we will do so. Right belief results in accurate intuitions and right action. Being rational is not one virtue among others but a pre-condition for all other virtues.