^1. See, e.g., 1 Peter Birks, English Private Law xxxv–xliii (2000); The Classification of Obligations (Peter Birks, ed. 1997); Hanoch Dagan, Legal Realism and the Taxonomy of Private Law, in Structure and Justification in Private Law: Essays for Peter Birks 103; Daniel Friedmann: The Creation of Entitlements through the Law of Restitution, in Structure and Justification in Private Law: Essays for Peter Birks 185 (Charles Rickett & Ross Grantham eds., 2008); Steve Hedley, The Shock of the Old: Interpretivism in Obligations, in in Structure and Justification in Private Law: Essays for Peter Birks 205 (Charles Rickett & Ross Grantham eds., 2008); Mitchell McInnes, Taxonomic Lessons for the Supreme Court of Canada, in Structure and Justification in Private Law: Essays for Peter Birks 77 (Charles Rickett & Ross Grantham eds., 2008); Richard Sutton, Restitution and the Discourse of System, in Structure and Justification in Private Law: Essays for Peter Birks 127 (Charles Rickett & Ross Grantham eds., 2008); Stephen Waddams, Contract and Unjust Enrichment: Competing Categories, or Complementary Concepts, in Structure and Justification in Private Law: Essays for Peter Birks 167 (Charles Rickett & Ross Grantham eds., 2008). See also Stephen Waddams, Dimensions of Private Law: Categories and Concepts in Anglo-American Legal Reasoning 1–22, 222–233 (2003) (discussing legal taxonomy and concluding that law is too complex and interrelated to accommodate logical classification). In an earlier essay, I offered tentative views on the subject of legal taxonomy, which I have since revised in substantial ways. See Emily Sherwin, Legal Positivism and the Taxonomy of Private Law, in Structure and Justification in Private Law: Essays for Peter Birks 103 (Charles Rickett & Ross Grantham eds., 2008).

^2. See, e.g., Peter Birks, Unjust Enrichment (2003); Peter Birks, The Foundations of Unjust Enrichment: Six Centennial Lectures (2002) Peter Birks, Introduction to the Law of Restitution 28–48 (1985); Birks, Peter, Unjust Enrichment and Wrongful Enrichment, 79 Tex. L. Rev. 1767, 1778–1779 (2001)Google Scholar; Jack Beatson, The Use and Abuse of Unjust Enrichment: Essays on the Law of Restitution (1991); Andrew Burrows, The Law of Restitution (2d ed. 2002); Hanoch Dagan, A Study of Private Law and Public Values (1997); Hanoch Dagan, The Law and Ethics of Restitution (2004); Philip Davenport & Christina Harris, Unjust Enrichment (1997); Lord Goff of Chieveley & Gareth Jones, The Law of Restitution 12 (6th ed. 2002); Steven Headley, Restitution: Its Division and Ordering (2001); Peter Jaffey, The Nature and Scope of Restitution: Vitiated Transfers, Imputed Contracts and Disgorgement (2000); Graham Virgo, The Principles of the Law of Restitution (1999); Essays on the Law of Restitution (Andrew Burrows ed., 1991); Robert Goff, The Search for Principle, in The Search for Principle 313, 324 (William Swadling & Gareth Jones eds., 1999); Understanding Unjust Enrichment (Jason W. Neyers, Mitchell McInnes, & Stephen G.A. Pitel eds., 2004); Restitution (Lionel Smith ed., 2000).

^3. See, e.g., Birks, English Private Law, supra note 1, at xxxv–xliii.

^4. Birks's taxonomic scheme is set forth succinctly in id. For additional statements, see, e.g., Birks, Unjust Enrichment, supra note 2, at 19–35; Peter Birks, Definition and Division, A Meditation on Institutes 3.13, in The Classification of Obligations 1, 35 (Peter Birks, ed. 1997); Birks, Unjust Enrichment and Wrongful Enrichment, supra note 2, at 1778–1779; Peter Birks, Equity in the Modern Law, 26 W. Austl. L. Rev. 1, 8 (1996). For an early version, see Birks, Introduction to the Law of Restitution, supra note 2, at 28–48.

^5. Birks, Definition and Division, supra note 4, at 21.

^6. See, e.g., Birks, English Private Law, supra note 1, at xli–xlii; Birks, Unjust Enrichment, supra note 2, at 20–21; Birks, Definition and Division, supra note 4, at 19–21; Birks, Unjust Enrichment and Wrongful Enrichment, supra note 2, at 1771–1772, 1777–1779.

^7. “The test of the validity of a taxonomy is precisely the question of whether any item within its purview can appear in more than one category pitched at the same level of generality. .–.–. It is no more possible for the selected causal event to be both an unjust enrichment and a tort than it is for an animal to be both an insect and a mammal.” Birks, Unjust Enrichment and Wrongful Enrichment, supra note 2, at 1780–1781. See also Nicholas J. McBride, The Classification of Obligations and Legal Education, in The Classification of Obligations 71 (Peter Birks, ed. 1997), (adopting a similar approach to legal taxonomy).

^8. See, e.g., Campbell, David, Classification and the Crisis of the Common Law, 26 J.L. Soc'y369 (1999) (criticizing “abstract doctrinal” attempts to classify law as indifferent to social and economic reality)Google Scholar; Samuel, Geoffrey, English Private Law: Old and New Thinking in the Taxonomical Debate, 24 Oxford J. Legal Stud.335 (2004) (proposing a classification scheme based on different forms of social relations in modern life)Google Scholar. See also Hanoch Dagan, Legal Realism and the Taxonomy of Law, in Structure and Justification: Essays for Peter Birks 147 (Charles Rickett & Ross Grantham eds., 2008) (writing on the question of taxonomy from a perspective inspired by American Legal Realism).

^9. See, e.g., Jaffey, Peter, Classification and Unjust Enrichment, 67 Mod. L. Rev. 1012 (2004) (arguing that unjust enrichment fails as a “justificatory” category of law)Google Scholar.

^10. One exception is the undergraduate legal text book initially compiled by Robert Summers: Robert S. Summers, Kevin M. Clermont, Robert A Hillman, Sheri Lynn Johnson, John J. Barcelo, III, & Doris Mae Provine, Law: Its Nature, Functions, and Limits ix–xi (3d ed. 1986).

^11. Warren, Samuel D. & Brandeis, Louis D., The Right to Privacy, 4 Harv. L. Rev. 193 (1890)Google Scholar; Fuller, L.L. & Perdue, William R. Jr., The Reliance Interest and Contract Damages (pts. 1 & 2), 46 Yale L.J.52, 373 (1936–1937)Google Scholar.

^12. By rules, I mean prescriptions that are general, in that they apply to classes of cases; determinate, in that they are readily and consistently understood by their intended audience; and authoritative, in that they are intended to dictate the outcome of all cases that fall within their terms. For discussion of the nature, function, and problems of “serious” rules, see Larry Alexander & Emily Sherwin, The Rules of Rules: Morality, Rules, and the Dilemmas of Law 53–95 (2001); Joseph Raz, The Morality of Freedom 57–62 (1986); Joseph Raz, The Authority of Law 16–19, 22–23, 30–33 (1979); Frederick Schauer, Playing by the Rules: A Philosophical Examination of Rule-Based Decision-Making in Life and Law 42–52, 77–134 (1991).

^13. The assumption that the meaning of a rule is the meaning intended by its author is likely to be controversial, particularly when rules are posited by multimember bodies such as courts or legislatures. This assumption is defended in Alexander & Sherwin, supra note 12, at 96–122.

^14. See Larry Alexander & Emily Sherwin, Demystifying Legal Reasoning 54–55 (2008). On canonicity as a characteristic of rules, see Schauer, supra note 12, at 68–72; Frederick Schauer, Prescriptions in Three Dimensions, 82 Iowa L. Rev. 911, 916–918 (1997).

^15. On generality as a characteristic of rules, see Schauer, supra note 12, at 17–37. A distinction is often drawn between rules and “standards,” which are not sufficiently determinate to dictate results without reference to controversial moral or evaluative propositions. See, e.g., Alexander & Sherwin, Rules of Rules, supra note 12, at 29–30; Cass R. Sunstein, Legal Reasoning and Political Conflict 27–28 (1996); Kaplow, Louis, Rules versus Standards: An Economic Analysis, 42 Duke L. Rev. 557 (1992)Google Scholar; Rose, Carol M., Crystals and Mud in Property Law, 40 Stan. L. Rev.577 (1988)Google Scholar.

^16. My analysis assumes that language is capable of carrying determinate meaning. See, e.g., Kent Greenawalt, Law and Objectivity 34–89 (1992); H.L.A. Hart, The Concept of Law 132–144 (1961); Schauer, supra note 12, at 53–68; Coleman, Jules L. & Leiter, Brian, Determinacy, Objectivity, and Authority, 142 U. Pa. L. Rev.549 (1992)Google Scholar; Solum, Lawrence B., On the Indeterminacy Crisis: Critiquing Critical Dogma, 54 U. Chi. L. Rev. 462 (1987)Google Scholar.

^17. “Abduction” is the term Charles Peirce uses to describe the process by which scientists move from observed data to tentative explanatory hypotheses. See Charles S. Peirce, Philosophical Writings of Peirce 150–156 (Justus Buchler ed., 1955); Brewer, Scott, Exemplary Reasoning: Semantics, Pragmatics, and the Rational Force of Legal Argument by Analogy, 109 Harv. L. Rev.945–949 (1996)Google Scholar.

^18. Ronald Dworkin's description of law follows this model. See Ronald Dworkin, Law's Empire 254–258 (1986); Ronald Dworkin, Taking Rights Seriously 115–118 (1978).

^19. Cf. Dworkin, Law's Empire, supra note 18, at 49, 228 (using the term “interpretation” in a special sense that refers to the insider's perspective toward law). Attribution of this kind is not, in my view, a form of “interpretation.” I assume throughout this essay that the meaning of any rule (or other text) is determined by the intentions of its authors. Interpretation, accordingly, is the process of discerning intended meaning. Any other ascription of meaning to a text, including attribution of rules and principles to legal materials generated by others, is not interpretation but authorship of a new text. This is, of course, a contested position, which I shall not attempt to defend here. For full discussion, see Alexander & Sherwin, Demystifying, supra note 14, at 131–141; Alexander & Sherwin, Rules of Rules, supra note 12, at 96–122.

^20. Dworkin, Law's Empire, supra note 18, at 240–250, 254–258; Dworkin, Taking Rights Seriously, supra note 18, at 22–31, 115–118 (1978).

^21. Judge Richard Posner's economic analysis of law might be viewed as a project of this kind. See Richard A. Posner, Economic Analysis of Law (7th ed. 2007).

^22. It has often been suggested that legal decisions can be sorted intuitively, at least by those who are well trained in law. See, e.g., Steven J. Burton, An Introduction to Law and Legal Reasoning 27–41 (1995); Anthony Kronman, The Lost Lawyer 109–162, 170–185, 209–225 (1995) Edward H. Levi, An Introduction to Legal Reasoning 1–6 (1948); Lloyd L. Weinreb, Legal Reason: The Use of Analogy in Legal Argument 78–90 (2005); Fried, Charles, The Artificial Reason of the Law, or: What Lawyers Know, 60 Tex. L. Rev. 35, 57 (1981)Google Scholar.

^23. For discussion of incompletely reasoned, or “System I,” responses in human psychology, see, generally, George Lakoff & Mark Johnson, Philosophy in the Flesh: The Embodied Mind and Its Challenge to Western Thought (1999); George Lakoff & Mark Johnson, Metaphors We Live By (1981); Howard Margolis, Patterns, Thinking, and Cognition: A Theory of Judgment 1–6, 42–86 (1987); Steven A. Sloman, Two Systems of Reasoning, in Heuristics and Biases: The Psychology of Intuitive Judgment 379 (Thomas Gilovick, Dale Griffin, & Daniel Kahneman eds., 2002); Haidt, Jonathan, The Emotional Dog and Its Rational Tail: A Social Intuitionist Approach to Moral Judgment, 4 Psychol. Rev.814 (2001)Google Scholar.

^24. This is true of any form of analogy. For an extended version of the argument that analogies are applications of general rules that pick out relevant similarities and differences among instances, see Alexander & Sherwin, Rules of Rules, supra note 12, at 64–88. Similar arguments appear in Melvin Aron Eisenberg, The Nature of the Common Law 84–87 (1988); Schauer, Playing by the Rules, supra note 12, at 183–187; Brewer, supra note 17, at 962–965; Peter Westen, On “Confusing Ideas”: Reply, 91 Yale L.J. 1153, 1163 (1982).

^25. See Haidt, supra note 23, at 818 (defining reasoning as conscious deliberation about choices in terms capable of articulation).

^26. Historical criteria for legal classification were more popular in the past than they are now. See, e.g., William A. Keener, A Treatise on the Law of Quasi-Contracts (1893); John Norton Pomeroy, Equity Jurisprudence (1881). The Restatement of Restitution initially divided the restitution into legal and equitable bodies of rules. See Restatement of Restitution pts. I & II (1937). In the new draft Restatement (Third) of Restitution and Unjust Enrichment, the law-equity distinction is dropped. See Restatement (Third) of Restitution and Unjust Enrichment (Discussion Draft 2000; Tentative Draft No. 1, 2001; Tentative Draft No. 2, 2002; Tentative Draft No. 3, 2004; Tentative Draft No. 4, 2005).

^27. I have borrowed the term “formal classification” from Peter Jaffey. See Jaffey, Classification, supra note 9, at 1015–1017.

^28. See notes 5–7 and accompanying text, supra. See also Jaffey, Classification, supra note 9, at 1017–1018 (characterizing Birks's taxonomy as formal). Another example of formal classification comes from Nicholas McBride, who argues that classification should “tell us all there is to know about our obligations in the most economical and accurate manner.” McBride, supra note 7, at 72. This requires a list in which no obligation listed is “an instance of another obligation on the list” and no two obligations on the list “are both instances of another obligation which is not on the list.” Id. at 74, 78. See also Stephen A. Smith, Taking Law Seriously, 50 U. Toronto L.J. 241, 254–255 (2000) (defending Birks's taxonomic scheme, which Smith describes as giving “moderate” recognition to the law's own organizational “self-understanding”).

^29. Birks, for example, was careful to include catchall categories to cover legal materials not otherwise accounted for. See Birks, English Private Law, supra note 1, at xlii; Birks, Unjust Enrichment, supra note 2, at 21–22; Birks, Definition and Division, supra note 4, at 19; Birks, Unjust Enrichment and Wrongful Enrichment, supra note 2, at 1769.

^30. This explains Birks's claim that the series “tort, contract, restitution” is mistaken because restitution is an outcome of rights rather than a causative event for rights. See Birks, Definition and Division, supra note 4, at 20–21; Birks, Unjust Enrichment and Wrongful Enrichment, supra note 2, at 1768–1769, 1771–1772.

^31. See McBride, supra note 7, at 71, 79 (listing, as one of six guidelines for classification of obligations, a requirement that obligations must not be “completely indeterminate”).

^32. As described above, Birks argues strenuously that the category of unjust enrichment must not include enrichments stemming from wrongs. See, e.g., Birks, Unjust Enrichment and Wrongful Enrichment, supra note 2, at 1780–1781.

^33. Function-based categories comprising primary rules of substantive law are likely to correspond directly to social activities or disputes found in society. Function-based categories comprising legal remedies or secondary rules for creation and application of law may incorporate notions of legal function that derive from the fundamental objective of settlement. For examples of function-based remedial taxonomy, see Douglas Laycock, Modern American Remedies 3 (3rd ed. 2002); Laycock, Douglas, The Scope and Significance of Restitution, 67 Tex. L. Rev. 267 (1997)Google Scholar.

^34. On the settlement function of law, see Alexander & Sherwin, Rules of Rules, supra note 12, at 11–15; Eisenberg, supra note 24, at 4–7; Joseph Raz, Ethics in the Public Domain 187–92 (1994).

^35. Summers et al., supra note 10. For prior versions, see Charles G. Howard & Robert S. Summers, Law: Its Nature, Functions, and Limits (1965); Robert S. Summers, Law: Its Nature, Functions, and Limits (2d ed. 1972). The third edition of the text represents a collaborative effort among Summers and a number of colleagues. A forthcoming fourth edition of the book retains the basic organizational scheme deployed in the third edition.

^36. On this point, it is interesting to note that a second section of the Summers text addresses what it terms the “Ends of Law.” Summers et al., supra note 10, at xi. This section discusses, by way of illustration, promotion of safety and promotion of equality, analyzing how different legal “techniques” (what I call functions) advance these ends. See id. at 636 (noting that law serves a wide variety of ends).

^37. See Schauer, Playing by the Rules, supra note 12, at 54–55 (discussing the relationship between rules and their justifications).

^38. Jaffey refers to this type of classification as “justificatory” classification. Jaffey, Classification, supra note 9, at 103–105.

^39. See Summers et al., supra note 10, at xi (listing public safety as one possible rationale for legal rules and processes).

^40. On interpretation, see note 19 and accompanying text, supra.

^41. A further difficulty is that there is very little in it for the taxonomer, whose job is simply to interpret and record the rationales adopted by other decision-makers.

^42. See Dworkin, Law's Empire, supra note 18, at 240–250, 254–258; Dworkin, Taking Rights Seriously, supra note 18, at 22–31, 115–118. Dworkin developed the notion of legal principles as a response to legal positivism. See Dworkin, Taking Rights Seriously, supra note 18, at 22. Courts, he argues, often rely on principles that are not posited rules but are derived from preexisting rules and decisions and are themselves part of the law.

^43. Typically, the material to be classified will be posited rules and judicial decisions. However, the taxonomer might first construct a set of attributed rules based on judicial decisions and narrower posited rules, then construct a further set of rationales that provide more general explanations for the attributed rules.

^44. In Dworkin's account of law, current judges formulate legal principles as standards for adjudicating particular disputes. A legal principle is not based on the intentions of the authorities who first announced the rules and decisions from which the principle is drawn, who may have had in mind a different principle or no principle at all. Rather than attempting to discern prior lawmakers' intentions, the current judge begins with the data of decisional outcomes and posited rules, formulates a principle or set of principles that meets the twin criteria of moral appeal and explanatory fit with legal data, then applies the principle to decide a pending dispute. See Dworkin, Law's Empire, supra note 18, at 240–250. A taxonomer classifying law according to attributed rationales would use the same method to provide judges with off-the-rack principles extracted from existing law and assembled in a comprehensive scheme.

As defined by Dworkin, legal principles must explain a certain portion of existing rules and decisions; beyond this, however, flawed rules and precedents can be discarded to achieve the morally best eligible principle. See Dworkin, Law's Empire, supra note 18, at 230–231, 239–250, 255; Dworkin, Taking Rights Seriously, supra note 18, at 118–123. Insofar as legal principles must be the morally best principles they can be, consistent with the requirement of fit, presumably all flawed materials must be discarded once the threshold is passed. See Alexander, Larry & Kress, Kenneth, Against Legal Principles, 82 Iowa. L. Rev.739, 756–757 (1997)Google Scholar; Kenneth J. Kress, Legal Reasoning and Coherence Theories: Dworkin's Rights Thesis, Retroactivity, and the Linear Order of Decisions, 72 Cal. L. Rev. 369, 380–381 (1984) Thus, assuming that at any point in time some number of prior legal rules and decisions will be mistaken, legal principles enable judges to improve on the raw material of existing law. In an imperfect world, however, legal principles cannot be ideal principles because they must satisfy the threshold requirement of fit with existing rules and decisions.

A final point about Dworkin's legal principles is that they do not operate as rules dictating the outcome of cases that fall within their terms. Instead, they have what Dworkin calls a “dimension of weight.” Dworkin, Taking Rights Seriously, supra note 18, at 26–27. Different legal principles may compete; when this occurs, the presiding judge balances them, giving each principle the weight it is due in the context of the case at hand.

^45. Restatement (Second) of Torts contents (1965).

^46. Restatement of Restitution, supra note 26.

^47. Cf. A. Mitchell Polisky, An Introduction to Law and Economics ix–xii, 157–162 (2003) (identifying themes in law and economics).

^48. Legal principles, as conceived by Dworkin, are inherently normative. Legal principles are not simply justifications for more determinate rules that govern the outcome of disputes. Instead, they operate as principles of law, which elaborate, extend, and sometimes override more particular legal rules and decisions. See Dworkin, Taking Rights Seriously, supra note 18, at 23–24, 29–39.

^49. Truly ideal rationales presumably would include a set of meta-rationales capable of resolving conflicts among more specific rationales.

^50. Formal legal concepts abstracted from the fact of particular cases were a prime target of American legal realists writing in the first half of the twentieth century. See, e.g., Cohen, Felix, Transcendental Nonsense and the Functional Approach, 35 Colum. L. Rev.809 (1935)Google Scholar. For a contemporaneous defense of formal legal concepts, see Gavit, Bernard C., A “Pragmatic Definition” of the “Cause of Action”?, 82 U. Pa. L. Rev. 129, 139–143 (1933) (arguing that “One who wishes to escape concepts must remain unborn”)Google Scholar. See, generally, Laura Kalman, Legal Realism at Yale: 1927–1960 at 3–44 (1986); Brian Leiter, Naturalizing Jurisprudence: Essays on American Legal Realism and Naturalism in Legal Philosophy 21–30 (2007).

^51. Birks speaks somewhat derisively of information storage, suggesting that the best tool for storage is alphabetization. Birks, English Private Law, supra note 1, at xlviii. But in fact information storage in quite important, and a sensible classification scheme makes it much easier to store and retrieve legal information.

^52. See id. at xlviii (“[w]ithout such a map of the law .–.–. it will be .–.–. impossible to pass the mind's eye over the many different locations in which difficult facts might fit”); Mitchell McInnes, supra note 1, at 87–88.

^53. Dworkin, discussing legal principles (which are, in effect, attributed rationales), appears to recognize that the principles he describes inevitably diverge from correct moral principles. In Dworkin's explication of the common law, judges are left to “weigh” competing principles and so resolve the conflicts among them case by case. See Dworkin, Taking Rights Seriously, supra note 18, at 26–27; note 44 supra.

^54. See, generally, Lakoff & Johnson, Philosophy in the Flesh, supra note 23; Lakoff & Johnson, Metaphors, supra note 23; Margolis, supra note 23.

^55. Analogical decision-making is sometimes depicted as a based on a special form of reasoning in which the decision-maker reasons directly from the outcome in one case to a parallel outcome in another case deemed to be analogous to the first. See, e.g., Burton, supra note 22, at 25–41; Raz, Authority of Law, supra note 12, at 201–206; Weinreb, supra note 22; Brewer, supra note 17, at 925, 925–929, 962–963; John F. Horty, The Result Model of Precedent, 10 Legal Theory 19 (2004); Grant Lamond, Do Precedents Create Rules?, 11 Legal Theory 1 (2005). See also Levi, supra note 22, at 1–6 (endorsing analogical decision-making but acknowledging its logical flaws); Karl N. Llewellyn, The Common Law Tradition: Deciding Appeals 77–87 (1960) (discussing “the leeways of precedent”); Karl N. Llewellyn, The Bramble Bush: On Our Law and Its Study 66–69 (1960) (same); Sunstein, supra note 15, at 62–100 (defending analogical decision-making as a form of “incompletely theorized” decision-making).

Others (including myself) have maintained that analogical decision-making is in fact a more or less conscious application of general rules. See, e.g., Alexander & Sherwin, Demystifying, supra note 14, at 66–87; Alexander & Sherwin, Rules of Rules, supra note 12, at 125–135; Richard A. Posner, The Problems of Jurisprudence 86–98 (1990); Schauer, Playing by the Rules, supra note 12, at 183–187; Alexander, Larry, Bad Beginnings, 145 U. Pa. L. Rev.57, 80–86 (1996)Google Scholar. See also Sherwin, Emily, A Defense of Analogical Reasoning, 66 U. Chi. L. Rev.1179 (1999) (defending the indirect benefits of a practice of analogical reasoning)Google Scholar.

^56. For fuller discussion of the reasons rule-makers may prefer that rule subjects obey the rules without considering whether the outcomes of the rules are consistent with the purposes and principles the rules are designed to carry out, see Alexander & Sherwin, Rules of Rules, supra note 12, at 53–95; Schauer, Playing by the Rules, supra note 12, at 128–34; Alexander, Larry, The Gap, 14 Harv. J.L. & Pub. Pol'y695 (1991)Google Scholar.

^57. See Raz, Morality of Freedom, supra note 12, at 57–62, 70–80 (discussing “exclusionary” reasons and the “normal justification” for rules).

^58. Perhaps this effect could be avoided by limiting access to the taxonomy of ideal reasons for law to those who design legal rules and withholding access from those who are expected to follow and apply legal rules. Yet the prospect of an esoteric legal taxonomy, hidden from most legal actors, poses a variety of practical and moral problems. See Alexander & Sherwin, Rules of Rules, supra note 12, at 86–91; Larry Alexander & Emily Sherwin, The Deceptive Nature of Rules, 142 U. Pa. L. Rev. 1191, 1211–1222 (1994). Not least of these practical difficulties is that in a system that accepts the doctrine of precedent, adjudicators are lawmakers.

^59. See Alexander, Lawrence & Bayles, Michael, Hercules or Proteus: The Many Theses of Ronald Dworkin, 5 Soc. Theory & Prac.267, 271–278 (1980)Google Scholar; Alexander & Kress, supra note 44, at 753–754.

^60. A preliminary difficulty is that there is no unique set of attributed rationales that best meet the criteria of overall virtue and fit with rules and decisions. It is not obvious how closely rationales must fit with existing rules and decisions to qualify as attributed rationales. See Dworkin, Law's Empire, supra note 18, at 255 (“different judges will set [the threshold of fit] differently”). The problem of fit is further complicated by the effect of new decisions: new decisions add to the body of law and thus alter the threshold of fit. See Alexander & Kress, supra note 44, at 756–757; Kress, supra note 44, at 380. In the absence of moral consensus, judges also might differ about which qualifying rationales are best. Thus, if the threshold of fit could be regularized, decision-makers still might not converge on the same set of attributed rationales. Different taxonomers might propose different schemes, and judges might not accept what taxonomers had to offer. If a particular scheme became prestigious enough to command substantial loyalty among judges, the problem of divergent rationales would be solved but the additional sources of inconsistency mentioned in the text would remain.

^61. See Dworkin, Taking Rights Seriously, supra note 18, at 26–27. Assigning weight to attributed rationales is a difficult if not impossible task. Attributed rationales are not morally ideal rationales; they are shaped in part by the requirement of fit with existing rules and past decisions and so are sure to incorporate some number of mistakes. Thus, when attributed rationales conflict, the question for judges is what moral weight to assign to reasons for decisions that are, by hypothesis, morally flawed. Correct moral principles cannot provide the answer, because the question itself makes no moral sense. Ultimately, therefore, the weight assigned to different principles depends on the judge's unregulated sense of which principle leads to the best result. See Alexander & Kress, supra note 44, at 756–757.

^62. See, generally, Hart, supra note 16, at 77–107.

^63. See, generally, id. at 121–133.

^64. Wide reflective equilibrium is the method of moral reasoning described by John Rawls, in which the reasoner tests a tentative moral principle against moral judgments about the correct outcome in particular instances within its scope. See John Rawls, A Theory of Justice 14–21, 43–53, 578–582 (1971); Rawls, John, Outline of a Decision Procedure for Ethics, 60 Phil. Rev.177 (1951)Google Scholar. The reasoner continues to adjust both the tentative principle and his or her more particular judgments, and also to seek independent confirmation from accepted background theories about the world, until an equilibrium is reached. As a method of justifying action or decisions, reasoning to wide reflective equilibrium is open to some devastating logical criticisms, but it is probably the best option we have. For explanation and defense of the method of reasoning to wide reflective equilibrium, see Daniels, Norman, Wide Reflective Equilibrium and Theory Acceptance, 76 J. Phil. 256 (1979)Google Scholar. For criticism, see, e.g., Haslett, D.W., What Is Wrong with Reflective Equilibria?, 37 Phil. Q.305 (1987)Google Scholar.

^65. Nor is an attributed rationale a form of moral principle incorporated by authoritative sources into law. Positivists disagree about whether moral principles can become legally authoritative by incorporation. See, e.g., Raz, Morality of Freedom, supra note 12, at 42–51 (defending exclusive legal positivism); Coleman, Jules L., Incorporationism, Conventionalism, and the Practical Difference Thesis, 4 Legal Theory381, 383–385 (1998) (defending inclusive legal positivism)Google Scholar; Shapiro, Scott J., On Hart's Way Out, 4 Legal Theory469 (1998) (defending exclusive legal positivism)Google Scholar.

^66. Examples furnished by the prior decisions can serve as a check on tentative decisional principles formulated by the judge. Using past cases to test decisional principles, however, is quite different from reasoning by analogy based on legal principles drawn from past cases. A judge constructing a legal principle takes the outcomes of past decisions as fixed starting points; a positivist judge deciding a case that falls outside the scope of prior posited rules uses the facts of past cases to test the moral soundness of an independent decisional principle. See Alexander, Bad Beginnings, supra note 55, at 66–68 (explaining the differences between analogical reasoning in law and reasoning to reflective equilibrium).

^67. For a discussion of epistemic authority, see Heidi M. Hurd, Moral Combat 63–65 (1999).

^68. See text accompanying note 44, supra.

^69. See Dworkin, Law's Empire, supra note 18, at 178–182, 217–219 (discussing “legislative integrity” and arguing that legislatures should avoid “checkerboard” laws in order to maintain “consistency in principle among the acts of the state,” even if legislation of this kind would achieve a fair compromise among competing political views). Dworkin warns, however, that considerations of justice can override considerations of consistency. Id. at 218–219.

^70. See, e.g., Alexander & Sherwin, Demystifying, supra note 14, at 109–114; Frederick Schauer, Do Cases Make Bad Law?, 73 U. Chi. L. Rev. 833, 893–906 (2006).

^71. See, generally, Judgment under Uncertainty: Heuristics and Biases 163 (Daniel Kahneman, Paul Slovic, & Amos Tversky eds., 1982); Heuristics & Biases, supra note 23; Scott Plous, The Psychology of Judgment and Decision Making (1993); Symposium: The Behavioral Analysis of Legal Institutions: Possibilities, Limitations, and New Directions, 32 Fla. St. U. L. Rev. 315 (2005).

^72. See, e.g., Amos Tversky & Daniel Kahneman, Availability: A Heuristic for Judging Frequence and Probability, in Judgment under Uncertainty: Heuristics and Biases 163 (Daniel Kahneman, Paul Slovic, & Amos Tversky eds., 1982); Plous, supra note 71, at 121–130; Schauer, Do Cases Make Bad Law?, supra note 70, at 894–895; Jeffrey J. Rachlinski,Bottom-up versus Top-down Lawmaking 73 U. Chi. L. Rev. 933, 942–943 (2006); Norbert Schwarz & Leigh Ann Vaughn, The Availability Heuristic Revisited: Ease of Recall and Content of Recall as Distinct Source of Information, in Heuristics & Biases: The Psychology of Intuitive Judgment 103 (Thomas Gilovick, Dale Griffin, & Daniel Kahneman eds., 2002).

^73. See, e.g., Paul Slovic, Melissa Finucane, Ellen Peters, & Donald G. MacGregor, The Affect Heuristic, in Heuristics and Biases: The Psychology of Intuitive Judgment 397 (Thomas Gilovick, Dale Griffin, & Daniel Kahneman eds., 2002); Rachlinski,supra note 72, at 943.

^74. For a fairly pessimistic assessment of the capacity of judges to distinguish between overruling unjustified rules and disregarding justified rules when they produce unpalatable results, see Alexander & Sherwin, Demystifying, supra note 14, at 114–117.

^75. Legal commentary pursuing a particular substantive approach to law, such as Richard Posner, Economic Analysis of Law, supra note 21, often proceeds in taxonomic form as a formal, functional, or reason-based classification of ideal legal rules. See id., contents at xxii. In work of this kind, however, taxonomy is usually incidental to the author's substantive project.

^76. See Birks, Definition and Division, supra note 4, at 34–35; Birks, Equity, supra note 4, at 4–5.

^77. See William A. Keener, A Treatise on the Law of Quasi-Contracts 16 (1893); Ames, J.B., The History of Disgorgement, 2 Harv. L. Rev. 53, 66, 69 (1888)Google Scholar; Kull, Andrew, James Barr Ames and the Modern History of Unjust Enrichment, 25 Oxford J. Legal Stud. 297 (2005)Google Scholar.

^78. Restatement of Restitution §1 (1937); see Seavey, Warren A. & Scott, Austin W., Restitution, 213 Legal Q. Rev.29 (1938)Google Scholar.

^79. See, e.g., John P. Dawson, Unjust Enrichment: A Comparative Analysis (1951); 1–4 George E. Palmer, The Law of Restitution (1978 & Supp. 1998); Dawson, John P., Restitution without Enrichment, 61 B.U. L. Rev.563 (1981)Google Scholar.

^80. See note 2 and accompanying text, supra.

^81. Draft Restatement, supra note 26.

^82. See, e.g., Peter Birks, Introduction, in Unjust Enrichment 18–25 (2003); Dagan, Law and Ethics of Restitution, supra note 2, at 12–18, 25–33; Birks, Definition and Division, supra note 4, at 21; Kull, Andrew, Rationalizing Restitution, 83 Cal. L. Rev. 1191, 1195–1196 (1995)Google Scholar; Sherwin, Emily, Restitution and Equity, 79 Tex. L. Rev. 2083, 2108–2112 (2001)Google Scholar. Cf. Dawson, Unjust Enrichment, supra note 79, at 4–5, 7–8, 24–26 (suggesting that unjust enrichment is too broad an idea to serve as a legal rule).

^83. The draft Restatement appears to reflect this view, although the Restatement also covers certain claims against defendants who have not received actual benefits, in order to undo the effects of invalid contracts. See draft Restatement, supra note 26, §1 cmt. a (Discussion Draft 2000) (“The source of a liability in restitution is the receipt of an economic benefit.”).

^84. Commentary that can be read this way includes Dan B. Dobbs, Law of Remedies, §4.1(2), at 557–558 (2d ed. 1993) (characterizing unjust enrichment as “the fundamental substantive basis for restitution”); Goff & Jones, supra note 2, at 12 (characterizing unjust enrichment as a “principle of justice which the law recognizes and gives effect to in a wide variety of claims”); 1 Palmer, supra note 79, §1, at 5 (stating that the idea of unjust enrichment “has played a creative role” in the development of restitution); Robert Goff, Appendix: The Search for Principle, in The Search for Principle 313, 324 (William Swadling & Gareth Jones eds., 1999) (favoring “the acceptance of a fully fledged principle of unjust enrichment .–.–. with the emphasis changing from the identification of specific heads of recovery to the identification and closer definition of the limits to a generalized right of recovery”).

^85. A particularly broad interpretation of this principle holds that unjust enrichment can and should assume the role once played by equity, enabling courts to make exceptions to otherwise applicable legal rules in all areas of law. See Peter Linzer, Rough Justice: A Theory of Restitution and Reliance, Contracts and Torts 2001 Wis. L. Rev. 695, 700–702, 773–775 (2001); Barry Nicholas, Unjustified Enrichment in the Civil Law and Louisiana Law, 36 Tul. L. Rev. 605, 607–610 (1962) (suggesting that unjust enrichment serves as a corrective to rules of law and as a gap-filler when rules fail). In other words, whenever one party benefits at another's expense from technical application of a rule, the principle of unjust enrichment authorizes courts to correct the outcome of the rule.

^86. At least, comparatively narrow definitions of restitution take this position. See Kull, supra note 82, at 1192.

^87. Some notable examples are Kossian v. American National Insurance Co., 254 Cal. App. 647 (1967) (allowing recovery for the value of work performed on land taken over by a mortgagee who also collected on the property); Sharp v. Kosmalski, 351 N.E. 2d 721 (N.Y. 1976) (allowing recovery of gifts bestowed in the course of a romance that ultimately failed).

^88. See draft Restatement, supra note 26, §§5–12 (Tentative Draft No. 1, 2001).

^89. Restatement of Restitution §187 (1937); see Dworkin, Taking Rights Seriously, supra note 18, at 23, 28–29.

^90. Draft Restatement, supra note 26, §28 (Tentative Draft No. 3, 2004); see Dagan, Law and Ethics of Restitution, supra note 2, at 165–183; Sherwin, Emily, Love, Money, and Justice: Restitution between Cohabitants, 77 U. Colo. L. Rev. 711 (2006)Google Scholar.

^91. See, e.g., Dobbs, supra note 84, §1.1, at 3; Laycock, supra note 30, Modern American Remedies at 15–16.

^92. See Emily Sherwin, Reparations and Unjust Enrichment, 84 B.U. L. Rev. 1443, 1459–1460. In cases of wrongdoing, a reduction in the defendant's welfare might be justified on retributive grounds, but not all claims of unjust enrichment involve wrongdoing, and in any event, legal responses to unjust enrichment are typically not measured according to culpability, as retributive responses must be. For discussion of the moral value of retribution, see, e.g., Robert Nozick, Philosophical Explanations 374–380 (1981) (explaining retribution as being necessary to connect the wrongdoer to moral values); Jean Hampton, The Retributive Idea, in Forgiveness and Mercy 111, 122–147 (Jeffrie G. Murphy & Jean Hampton eds., 1988) (explaining retribution as a means of affirming the value of the victim); Michael S. Moore, The Moral Worth of Retribution, in Responsibility, Character, and the Emotions 179 (Ferdinand Schoeman ed., 1987) (defending retribution as a good in itself, on grounds of moral desert); Herbert Morris, Persons and Punishment, 52 Monist 475, 482–486 (1968) (explaining retribution as a balance of moral accounts).