Appeals to normative facts cannot explain how those facts count as reasons or motivate rational agents.

Korsgaard’s case for constructivism parallels Kant’s argument for the autonomy of practical reason, as Rawls reconstructs it. It starts by objecting that substantive realism fails to respond to the skeptical challenge that there really are no reasons to be moral. This is because realism simply assumes the existence of objective standards for morality without offering a rational basis for them; hence the realist affirms what the skeptic denies. As a consequence, the realist also fails to show why