While there has been a revival of interest in the “social contract,” the idea is a very ancient one, with more than a nod given to it in Plato’s Republic and in the writings of Epicurus, for example, and then again in a great explosion of the concept in the 17th and 18th centuries. We also encounter considerable variation, and controversy, in the idea: first, in what it is an idea about, and second, in just what the “contractual” element is supposed to consist.

Regarding the first point, the two main options are (1) that it should be exclusively a theory of politics in particular and (2) that it should be more generally a theory about moral relations in society as a whole. In the first ...