Thomas Hobbes's theory of social contract has received much attention and commentary from scholars. Less attention has been given to his theory of contract as a theory of private law contracts. But it has not been noted by scholars that underlying Hobbes's treatment of social and private law contracts there is a theory of an a priori notion of contract, that abstracts from any instance of contract, including the social contract, and that therefore claims a universality social and private contracts theories don't do. In this paper we expose Hobbes's theory of an a priori notion of contract that serves to organize sense experience, and we show how Hobbes justifies the tenets of that theory.