Contract and promise
A contract theory is an attempt both to make normative sense of contract law as an institutional type and to come up with criteria for the evaluation of the law of any particular place. There is no precise rule telling us how far the prescriptions of a theory can deviate from actually existing contract law and still be a theory of contract — rather than a political proposal to replace contract law with something else. But we can say roughly that contract theory aims to provide normative foundations for the type of legal institution that enforces (some) agreements and unilateral commitments. Having provided an account of the point of having an institution of that general kind, the theory can then be used to evaluate existing examples.