Going beyond the debate between positivists and proponents of natural law, there is a controversy over whether there can or ought to be “philosophy in law” (i.e., whether anything within the subject-matter of philosophy can also become part of the subject-matter of law). According to Luhmann's autopoietic theory, law is a normatively closed system and accordingly remains completely independent from philosophy. Dworkin, on the other hand, asserts that constitutional law depends for its coherence and integrity on being encompassed within a particular political philosophy. This essay approaches “philosophy in law” from a functional rather than a legitimating perspective, and concludes against both Luhmann and Dworkin that the integration of philosophy in law is interstitial and limited. The consequence of this for law's validity and legitimacy is a likely increase in contestation and contestability. The essay concludes that by embracing pluralism as a philosophy, one can reduce and better manage contestability without ever becoming able to eliminate it.