Reflective equilibrium is a state of balance or coherence among a set of beliefs arrived at by a process of deliberative mutual adjustment among general principles and particular judgments. Although he did not use the term, philosopher Nelson Goodman introduced the method of reflective equilibrium as an approach to justifying the principles of inductive logic. The term 'reflective equilibrium' was coined by John Rawls and popularized in his A Theory of Justice as a method for arriving at the content of the principles of justice.
Rawls argues that human beings have a "sense of justice" which is both a source of moral judgment and moral motivation. In Rawls's theory, we begin with "considered judgments" that arise from the sense of justice. These may be judgments about general moral principles (of any level of generality) or specific moral cases. If our judgments conflict in some way, we proceed by adjusting our various beliefs until they are in "equilibrium," which is to say that they are stable, not in conflict, and provide consistent practical guidance. Rawls argues that a set of moral beliefs in ideal reflective equilibrium describes or characterizes the underlying principles of the human sense of justice.
An example of the method of reflective equilibrium may be useful. Suppose Zachary believes in the general principle of always obeying the commands in the Bible, and mistakenly thinks that these are completely encompassed by every Old Testament command. Suppose also that he thinks that it is not ethical to stone people to death merely for being Wiccan. These views may come into conflict (see Exodus 22:18, but see John 8:7). If they do, Zachary will then have several choices. He can discard his general principle in search of a better one (for example, only obeying the Ten Commandments), modify his general principle (for example, choosing a different translation of the Bible, or letting Jesus' teaching from John 8:7 "If any of you is without sin, let him be the first to cast a stone" override the Old Testament command), or change his opinions about the point in question to conform with his theory (by deciding that witches really should be killed). Whatever the decision, he has moved toward reflective equilibrium.
Reflective equilibrium serves an important justificatory function within Rawls's political theory. The nature of this function, however, is disputed. The dominant view, best exemplified by the work of Norman Daniels and Thomas Scanlon, is that the method of reflective equilibrium is a kind of coherentist method for the epistemic justification of moral beliefs. However, in other writings, Rawls seems to argue that his theory bypasses traditional metaethical questions, including questions of moral epistemology, and is intended instead to serve a practical function. This provides some motivation for a different view of the justificatory role of reflective equilibrium. On this view, the method of reflective equilibrium serves its justificatory function by linking together the cognitive and motivational aspects of the human sense of justice in the appropriate way.
Rawls argues that candidate principles of justice cannot be justified unless they are shown to be stable. Principles of justice are stable if, among other things, the members of society regard them as authoritative and reliably compliant with them. The method of reflective equilibrium determines a set of principles rooted in the human sense of justice, which is a capacity that both provides the material for the process of reflective equilibration and our motivation to adhere to principles we judge morally sound. The method of reflective equilibrium serves the aim of defining a realistic and stable social order by determining a practically coherent set of principles that are grounded in the right way in the source of our moral motivation, such that we will be disposed to comply with them. As Fred D'Agostino puts it, stable principles of justice will require considerable "up-take" by the members of society. The method of reflective equilibrium provides a way of settling on principles that will achieve the kind of "up-take" necessary for stability.
Reflective equilibrium is not static, though Rawls allows for provisional fixed points; it will change as the individual considers his opinions about individual issues or explores the consequences of his principles.
Rawls applied this technique to his conception of a hypothetical original position from which people would agree to a social contract. He arrived at the conclusion that the optimal theory of justice is the one to which people would agree from behind a veil of ignorance, not knowing their social positions.