Social cooperation has always been an important concept in Rawls’s political philosophy. In A Theory of Justice he conceives of society as “a cooperative venture for mutual advantage,” whose members acknowledge “certain rules of conduct as binding,” rules that “specify a system of cooperation designed to advance the good of those taking part in it” (TJ 4). In his later writings, however, social cooperation becomes even more central. The “idea of a society as a fair system of social cooperation over time from one generation to the next,” he tells us, is the “most fundamental idea” in his conception of justice as fairness (JF 5). Indeed, it is even built into his conception of political philosophy, whose first task is to narrow “the divergence of philosophical and moral opinion at the root of divisive political differences” so that “social cooperation on a footing of mutual respect among citizens can still be maintained” (JF 2).

Social cooperation is best seen by contrast to “socially coordinated activity – for example, activity coordinated by orders issued by an absolute central authority” (JF 6). The key point is that all social life involves a division of social labor, not merely in the narrowly economic sense epitomized by Adam Smith’s pin factory, but in a broader sense of differentiated social roles, each with its own responsibilities and powers.

The notion of a well-ordered society has been a central idea in Rawls’s thinking about justice at least since his great opus, A Theory of Justice, in which it plays a crucial role. The core idea of a well-ordered society is a society whose citizens all accept the same conception of justice, its institutions actually conform to that conception, and both of these facts are publicly known. By envisioning different societies, each organized by a particular conception of justice, and so each “well-ordered” in terms of that conception, we can get some traction on the question of which of the many conflicting conceptions of justice is the most reasonable, is the most worthy of adoption. Over time the idea of a well-ordered society has come to play an increasingly important role. In Justice as Fairness: A Restatement, Rawls presents the idea of a “well-ordered society” as one of two “companion fundamental ideas” to the “most fundamental idea in [his] conception of justice,” which is the “idea of society as a fair system of social cooperation” (JF 5). (The second companion idea is the “idea of citizens…as free and equal persons.”) Rawls offers a definition or explication of this term in each of his major writings, and in many papers as well. Though there are slight differences among these formulations, the core idea has remained fairly stable.

The idea of reciprocity has always been central to Rawls’s thinking about justice. In A Theory of Justice he writes that reciprocity is “implicit in the notion of a well-ordered society” (TJ 13). But in his later work it plays an explicitly foundational role. Indeed, that work is based on the “fundamental idea … of society as a fair system of cooperation over time, from one generation to the next” (PL 14), and reciprocity is integral to that idea because “Fair terms of cooperation specify an idea of reciprocity: all who are engaged in cooperation and who do their part as the rules and procedure require, are to benefit in an appropriate way as assessed by a suitable benchmark of comparison” (PL 16). Reciprocity differs from pure altruism, in which one is motivated simply “by the general good,” and “mutual advantage,” under which everyone’s interests are advanced relative to a merely existing situation or baseline. In a relationship of reciprocity, by contrast to mutual advantage, “everyone benefits judged with respect to an appropriate benchmark of equality” (PL 17).

Rawls introduced the phrase “political liberalism” to distinguish his own account of liberalism from what he called “comprehensive liberalism,” which denotes a liberal theory of politics rooted in a “comprehensive doctrine,” or a doctrine that is at least partially comprehensive. As Rawls explains, modern political thinkers “hoped to establish a basis of moral knowledge independent of ecclesiastical authority” that could be grasped by ordinary people, and that could be used “to develop the full range of concepts and principles in terms of which to characterize the requirements of moral life” (PL xxvi). In this type of theory, political principles are derived from the “basis of moral knowledge,” suggesting that in this view political philosophy is “applied moral philosophy” (JF 14). Although Rawls does not explicitly mention it, many liberals such as J. S. Mill envisioned that the establishment of liberal forms of politics and related changes in the law and economic relationships would be accompanied by a cultural transformation, as society became increasingly secular and religious and other traditional sources of values and identity would atrophy. The widespread acceptance of liberal comprehensive doctrines, then, would support liberal political institutions.

Political legitimacy does not figure in Rawls’s early work, notably in A Theory of Justice. In Theory he does make use of the broader idea of “legitimate expectations” that citizens come to have when they participate in a social practice, but his main concern is to specify the principles of justice to which the basic structure of society, including the structure of political authority, must conform. Not surprisingly, legitimacy is a major theme of his later work, in which he develops his account of political liberalism. Legitimacy figures in two principal ways: one is the idea of “legitimate law,” and the other is the “principle of legitimacy,” or what he sometimes calls the “liberal (or democratic) principle of legitimacy.” Both of these uses are related to the conventional notion of a legitimate authority as having the right to exercise political power to make a law or decide policy, with those subject to that authority having at least a presumptive obligation to obey. For Rawls, legitimate law results from a properly constituted political process, and “is politically (morally) binding on [one] as a citizen and is to be accepted as such” (CP 578). Such a properly constituted process must accord with the principle of legitimacy.

The principle of legitimacy arises directly from the defining problem of political liberalism, which is to explain how a society can be well-ordered by a conception of justice when its citizens adhere to conflicting comprehensive doctrines, each of which may give rise to its own conception of justice. Political liberalism solves that problem by proposing a political conception of justice that all reasonable citizens may accept because it is not based upon the concepts or principles of any particular comprehensive doctrine.

As the idea of an ultimate foundation for moral and political beliefs has become increasingly implausible, theorists have turned to “discourse” to provide a basis on which to defend the legitimacy of social and political practices. The turn to discourse, which includes but is not limited to communicative ethics, is in part a move from a substantive to a procedural conception of moral and political theory. Rather than providing values grounded in an account of human nature or reason, discourse-based approaches offer a set of procedures that, if followed, would yield principles legitimating social practices and institutions. The fundamental intuition underlying the move to discourse is the ideal of a moral community, one whose norms and practices are fully acceptable to those subject to them, a society based not on imposition, but on the agreement of free and equal persons.

Jürgen Habermas has presented one of the most powerful accounts of a discourse-based morality,- it is grounded in an understanding of practical reason which explains how the validity of norms can be tested, thereby demonstrating their cognitive character. According to Habermas, valid norms can be freely accepted by all of the individuals who are affected by them. Thus, a society whose institutions and practices were governed by valid norms would instantiate the ideal of a moral community.