In the development of post-kantian philosophy there is an ongoing discussion about the relationship between reason and real external world. This essay argues for Kant's insistence on the external character of law as a necessary moment in the realization of rational legislation as such showing furthermore the origin of Kant's terminology concerning law. The aim of this article is to show that, as per Kant, law as exteriority of practical legislation is a main component of reason becoming real in external world.