On The Concept of Law (1961), ch. 6, "Foundations of a Legal System," on Hart's concept of a rule of recognition that ultimately determines what will count as a law in a given society. This ends up being more complicated than merely "The Constitution," but the action itself of officials respecting, obeying, and enforcing that Constitution. If you're not hearing the full version of this part of the discussion, sign up via one of the options described at partiallyexaminedlife.com/support.