The most intuitive conception of law regards it as derivative from or subordinate to ideal morality. It holds that if law is morally justified, then its justification necessarily derives from the fact that it reflects or mirrors ideal morality. I distinguish between practical idealness and epistemic idealness. Whereas practical idealness focuses on publicity and perfect compliance, epistemic idealness refers to (possible) perfect knowledge of information that is necessary to follow and apply rules.

Now, law is capable of reflecting or mirroring ideal moral principles only if it can meet the epistemic demands of ideal moral theory. In fact, compliance with ideal moral principles presupposes factual knowledge that often is not easily available. In order to perform this epistemic function, law must include the functioning of reliable truth- searching institutions. In most legal systems, courts are both truth-searching and conflict-settling institutions.

Traditional areas of international law lack an institutional system endowed with factfinding and truth-ascertaining powers. Therefore, international legal rules must refer to incontestable conditions of application because such rules must be self-applied by states without the possible epistemic arbitration of a neutral party. Consider jus in bello, that is, the system of legal rules that regulate permissible behavior in war. Jeff McMahan has argued that the theory of just war should differentiate between just and unjust combatants in terms of their fulfillment of jus ad bellum requirements, equating just combatants with civilians. But he also observes that this ideal moral view cannot be translated into jus in bello in the present circumstances because there are no judicial bodies that can assess and establish the jus ad bellum status of belligerent parties. This means that, in the absence of truth-ascertaining institutions, ideal moral principles are irrelevant for international law whenever they contemplate contestable conditions of application.