Every legal system must decide how to distinguish between agreements that are enforceable and those that are not. Formal bargains in the marketplace and casual promises in a social setting mark the two extremes, but many hard cases lie between. When gaps are left in a contract, how should courts fill them? What does it mean to say that an agreement is legally enforceable? If someone breaks a legally enforceable contract, what consequences follow?

For 150 years, legal scholars have debated whether a set of coherent principles provide answers to such basic questions. Oliver Wendell...