Reason, Properly Understood, Is Public and Sharable
…if our social nature is deep, in the sense that it is the nature of our reasons that they are public and shareable, then justifications of morality can and should appeal to it. So the kind of argument we need here is not one that shows us that our private reasons somehow commit us to public ones, but one that acknowledges that our reasons were never more than incidentally private in the first place. To act on a reason is already, essentially, to act on a consideration whose normative force may be shared with others.
The Sources of Normativity, p. 136