But the NAP cannot justify itself without an objective authority who makes such a principle intelligible and necessary. Who is that authority, according to proponents of the NAP?
What makes such a principle intelligible is the works of people like Murray Rothbard who observe the natures of man and use their reason to put forth a consistent doctrine of ethics. People may some day adopt this doctrine just as they have adopted other forms of guidance for what is right and wrong. Rejecting the coercive monopoly state just happens to be particularly radical considering how we're all raised with the assumption that individual sovereignty and private property rights must be violated to ensure the state's existence.