The notion of social union is central to A Theory of Justice in which the just society is described as a “social union of social unions” (TJ 462). In effect, the idea expresses Rawls’s view of “human sociability” and is meant to deflect charges that his contract doctrine cannot account for the “value of community” (TJ 456).

According to Rawls, our social nature is all too often described in “a trivial” fashion (TJ 458). One repeatedly hears, for instance, that we are “social creatures” or that society is necessary for human life, for acquiring language and certain interests, even for our ability to think. These facts are not trivial but to claim that our social nature consists in them is inadequate, for all these facts are equally true of a group of egoists. Egoists too cannot learn to speak nor develop their selfish ways outside specific human communities. Genuine human sociability entails something more; it requires social union.

Rawls’s idea of social union is drawn up in explicit contrast to that of “private society” or that form of social organization distinguished by two features: individuals comprising it have their own exclusive ends, either competing or independent, but in any case not complementary, and they view their social relations and institutions as means to these private ends (TJ 458). The “natural habitat” of this idea is the economic theory of competitive markets, and it can already be found in the thought of A. Smith and in Hegel’s notion of civil society (TJ 459 n.4).