Like Finnis, Lon Fuller (1964) rejects the conceptual naturalist idea that there are necessary substantive moral constraints on the content of law. But Fuller, unlike Finnis, believes that law is necessarily subject to a procedural morality. On Fuller’s view, human activity is necessarily goal-oriented or purposive in the sense that people engage in a particular activity because it helps them to achieve some end. Insofar as human activity is essentially purposive, according to Fuller, particular human activities can be understood only in terms that make reference to their purposes and ends. Thus, since lawmaking is essentially purposive activity, it can be understood only in terms that explicitly acknowledge its essential values and purposes:
The only formula that might be called a definition of law offered in these writings is by now thoroughly familiar: law is the enterprise of subjecting human conduct to the governance of rules. Unlike most modern theories of law, this view treats law as an activity and regards a legal system as the product of a sustained purposive effort (Fuller 1964, 106).
To the extent that a definition of law can be given, then, it must include the idea that law’s essential function is to “achiev[e] [social] order through subjecting people’s conduct to the guidance of general rules by which they may themselves orient their behavior” (Fuller 1965, 657).
Fuller’s functionalist conception of law implies that nothing can count as law unless it is capable of performing law’s essential function of guiding behavior. And to be capable of performing this function, a system of rules must satisfy the following principles:
  • (P1) the rules must be expressed in general terms;
  • (P2) the rules must be publicly promulgated;
  • (P3) the rules must be prospective in effect;
  • (P4) the rules must be expressed in understandable terms;
  • (P5) the rules must be consistent with one another;
  • (P6) the rules must not require conduct beyond the powers of the affected parties;
  • (P7) the rules must not be changed so frequently that the subject cannot rely on them; and
  • (P8) the rules must be administered in a manner consistent with their wording.
On Fuller’s view, no system of rules that fails minimally to satisfy these principles of legality can achieve law’s essential purpose of achieving social order through the use of rules that guide behavior. A system of rules that fails to satisfy (P2) or (P4), for example, cannot guide behavior because people will not be able to determine what the rules require. Accordingly, Fuller concludes that his eight principles are “internal” to law in the sense that they are built into the existence conditions for law.
These internal principles constitute a morality, according to Fuller, because law necessarily has positive moral value in two respects: (1) law conduces to a state of social order and (2) does so by respecting human autonomy because rules guide behavior. Since no system of rules can achieve these morally valuable objectives without minimally complying with the principles of legality, it follows, on Fuller’s view, that they constitute a morality. Since these moral principles are built into the existence conditions for law, they are internal and hence represent a conceptual connection between law and morality. Thus, like the classical naturalists and unlike Finnis, Fuller subscribes to the strongest form of the Overlap Thesis, which makes him a conceptual naturalist.
Nevertheless, Fuller’s conceptual naturalism is fundamentally different from that of classical naturalism. First, Fuller rejects the classical naturalist view that there are necessary moral constraints on the content of law, holding instead that there are necessary moral constraints on the procedural mechanisms by which law is made and administered: “What I have called the internal morality of law is … a procedural version of natural law … [in the sense that it is] concerned, not with the substantive aims of legal rules, but with the ways in which a system of rules for governing human conduct must be constructed and administered if it is to be efficacious and at the same time remain what it purports to be” (Fuller 1964, 96- 97).
Second, Fuller identifies the conceptual connection between law and morality at a higher level of abstraction than the classical naturalists. The classical naturalists view morality as providing substantive constraints on the content of individual laws; an unjust norm, on this view, is conceptually disqualified from being legally valid. In contrast, Fuller views morality as providing a constraint on the existence of a legal system: “A total failure in any one of these eight directions does not simply result in a bad system of law; it results in something that is not properly called a legal system at all” (Fuller 1964, 39).
Fuller’s procedural naturalism is vulnerable to a number of objections. H.L.A. Hart, for example, denies Fuller’s claim that the principles of legality constitute an internal morality; according to Hart, Fuller confuses the notions of morality and efficacy:
[T]he author’s insistence on classifying these principles of legality as a “morality” is a source of confusion both for him and his readers…. [T]he crucial objection to the designation of these principles of good legal craftsmanship as morality, in spite of the qualification “inner,” is that it perpetrates a confusion between two notions that it is vital to hold apart: the notions of purposive activity and morality. Poisoning is no doubt a purposive activity, and reflections on its purpose may show that it has its internal principles. (“Avoid poisons however lethal if they cause the victim to vomit”….) But to call these principles of the poisoner’s art “the morality of poisoning” would simply blur the distinction between the notion of efficiency for a purpose and those final judgments about activities and purposes with which morality in its various forms is concerned (Hart 1965, 1285-86).
On Hart’s view, all actions, including virtuous acts like lawmaking and impermissible acts like poisoning, have their own internal standards of efficacy. But insofar as such standards of efficacy conflict with morality, as they do in the case of poisoning, it follows that they are distinct from moral standards. Thus, while Hart concedes that something like Fuller’s eight principles are built into the existence conditions for law, he concludes they do not constitute a conceptual connection between law and morality.
Unfortunately, Hart overlooks the fact that most of Fuller’s eight principles double as moral ideals of fairness. For example, public promulgation in understandable terms may be a necessary condition for efficacy, but it is also a moral ideal; it is morally objectionable for a state to enforce rules that have not been publicly promulgated in terms reasonably calculated to give notice of what is required. Similarly, we take it for granted that it is wrong for a state to enact retroactive rules, inconsistent rules, and rules that require what is impossible. Poisoning may have its internal standards of efficacy, but such standards are distinguishable from the principles of legality in that they conflict with moral ideals.
Nevertheless, Fuller’s principles operate internally, not as moral ideals, but merely as principles of efficacy. As Fuller would likely acknowledge, the existence of a legal system is consistent with considerable divergence from the principles of legality. Legal standards, for example, are necessarily promulgated in general terms that inevitably give rise to problems of vagueness. And officials all too often fail to administer the laws in a fair and even-handed manner even in the best of legal systems. These divergences may always be prima facie objectionable, but they are inconsistent with a legal system only when they render a legal system incapable of performing its essential function of guiding behavior. Insofar as these principles are built into the existence conditions for law, it is because they operate as efficacy conditions and not because they function as moral ideals.

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