This brings us to what, I take it, has to be the real objection to the naturalist about normativity: namely, that he has not explained real normativity—that is, the bindingness of standards independent of our attitudes—and that explaining the real normativity of reasons is indispensable for creatures like us when we are trying to figure out what to do (or believe).
The claim that it is “irrational for one to do a” means, for the naturalist, nothing more than some people or even all people might feel that you should not do a.
The NeoHumean naturalist has not explained real normativity, as Scanlon complains, because real normativity does not exist: that is the entire upshot of the naturalist view. There are no reasons whose existence and character is independent of human attitudes; there are only human attitudes which lead us to “talk the talk” of reasons, to feel that we should act one way rather than another. And if real normativity does not exist, if only feelings of inclination and aversion, compulsion and avoidance, actually exist, then that means that all purportedly normative disputes bottom out not in reasons but in the clash of will or affect.
If we have no real reason to believe the same or act the same, and thus we may not believe the same or act the same, given that our underlying psychological states (our attitudes) vary, what follows? What follows is basically what Ayer and Stevenson correctly diagnosed not quite a century ago: where people share attitudes, reasoning about what one ought to do and what one ought to believe is possible; where people do not share attitudes, reasoning is not possible and only force prevails in a dispute, whether that is the rhetorical force of producing a change in attitudes by whatever means are effective or the physical or lawful force of suppressing contrary attitudes. An agent deciding what to do or what to believe is in the grips of particular normative attitudes, some practical and some theoretical, and has no reason to discount them since after all they are her attitudes—although, as Nietzsche noticed, she might discount them if she were in the grips of a non-naturalistic view of what had to be true of her attitudes for them to move her, that is, if she thought they had to be something more than her attitudes.
Nietzsche’s Moral Psychology, Ch. 4