In “Contractualism and Utilitarianism,” Scanlon first presents his contractualist theory of moral rightness. According to that theory, elaborated in his subsequent work, conduct is morally wrong just in case it conflicts with “any set of principles for the general regulation of behavior that no one could reasonably reject as a basis for informed, unforced general agreement.” Lying and stealing, for example, are wrong because principles that permit lying or stealing can reasonably be rejected as a basis for such agreement. Intuitively, when you lie or steal, you act in ways that you cannot justify to others.
Scanlon's contractualism shares with other contract theories the concern about justifiability to others. A distinctive feature of Scanlon's view is that, in conducting such mutual justification, the contracting parties themselves use a normative notion of reasonableness. When we reason about right and wrong, we ask ourselves whether a proposed principle is reasonable for others to accept or reject. The notion of reasonableness is distinct from and irreducible to rationality. When I assess a proposed principle for the general regulation of behavior, I do not ask whether compliance with the principle is rationally advantageous – an effective means for achieving my aims. Instead, I ask whether the principle is reasonable to accept or reject.
As Scanlon emphasizes in “Contractualism,” moral contractualism contrasts on this point with Rawls's social-contract theory of justice. In TJ, Rawls observes that different contract theories – those developed by Hobbes, Locke, Rousseau, and Kant, for example – all employ an idea of justification via universal agreement in an “initial situation.” They differ in how they interpret that initial situation. In Rawls's preferred interpretation of the initial situation – the original position – the parties, reasoning under informational constraints, make a rational choice of principles. Each person asks what the most effective way is of advancing his or her good. Parties in the Rawlsian initial choice situation thus make no use of the notion of reasonableness. The original position relies instead on what I will be calling the Rational Advantage Model.
To be clear, Rawls does not suppose that people in a just society (or any society) act solely on the basis of judgments of rational advantage. In a just society, people act in part on the basis of their sense of justice, as specified by a set of principles of justice.