Contract law has neither a complete descriptive theory, explaining what the law is, nor a complete normative theory, explaining what the law should be. These gaps are unsurprising given the traditional definition of contract as embracing all promises that the law will enforce. Even a theory of contract law that focuses only on the enforcement of bargains must still consider the entire continuum from standard form contracts between firms and consumers to commercial contracts between business firms. No descriptive theory has yet explained a law of contract that comprehends such a broad domain. Normative theories that are grounded in a single norm -- such as autonomy or efficiency -- also have foundered over the heterogeneity of contractual co...