A law of nature is not a law in the normal sense of the word, but a generalisation about observed regularities. The laws governing the activities of people in societies, and attributed to the authorship of gods, kings or parliaments, are in some way stipulative or imperative. But in what way? What is law, and what justifies it?
Philosophical debate about the nature of law is called "jurisprudence," and most discussions of it begin with John Austin and his The Province of Jurisprudence Determined (1832). He defined law as a command given by a sovereign and enforced by a sanction, where a "sovereign" is an identified common superior which the majority of a given society's members habitually obey, and which is itself supreme. This "command" theory of law seems obvious, and Austin was not its first exponent; Jean Bodin in Six Books of the Republic (1576), Thomas Hobbes in Leviathan (1651) and Jeremy Bentham in A Fragment on Government (composed 1776) took similar views. Austin is however its first systematic proponent.
A crucial feature of Austin's view is its rejection of the concept of "natural law" (not the same thing, note, as "laws of nature" in the physicist's sense). It had long been argued that man-made laws are valid only if they do not conflict with a higher law, such as the law of God; the validity of law, in other words, is related to its moral content. Austin rejected this, saying that if a law is commanded by a sovereign, it is ipso facto valid, irrespective of its morality. This is called "positivism": it holds that the question whether a law is a law differs from the question whether it is a morally acceptable.
It was over a century before Austin's theory was challenged. But it was challenged brilliantly, by HLA Hart in The Concept of Law (1961). Hart showed that actual real-life laws are not like orders backed by threats, as the command theory states. Nor is Austin's theory of unlimited sovereignty satisfactory, because legislative bodies or individuals are typically constrained in their powers-say, by a constitution; and can in certain circumstances be checked by external agencies-say, a court; and might ultimately owe their authority to something else-say, an electorate. Hart's view of the nature of a legal system turns instead on the idea of "rules of recognition," which enable us to know what laws are by identifying their sources (in parliament, courts, constitution).
But Hart accepted Austin's positivism, which led to a celebrated debate with LH Fuller. Something like the idea of "natural law," whose first traces are evident in Plato and Aristotle, but which flowered in Stoic works such as Cicero's On Duties and Marcus Aurelius's Meditations, and later assumed a central place in Roman Catholic legal thinking under the influence of Aquinas's Summa Theologiae, was invoked by German prosecutors after 1945 when they tried people whose defence was that they had acted in obedience to Nazi laws. The courts held that those laws were "contrary to the sound conscience and sense of justice of all decent human beings." Fuller applauded the German courts' decision, arguing that a legal system must have an "inner morality" to justify it, where this phrase denotes possession of such features as generality, consistency and clarity. Hart responded that it confused matters to say that a law is not a law if it is evil, rather than recognising it as a law but an evil one which must be opposed or changed.
Austin's reason for rejecting natural law, as with David Hume before him in An Enquiry Concerning the Principles of Morals (1751), was that it attempts to deduce an "ought"-a statement of what should be done-from an "is"-a description of natural facts about the nature of human beings. Others pointed to the opposing claims made by the concept of natural law; for example, John Locke held that democracy is enjoined by natural law, whereas Robert Filmer argued the contrary on the ground that God, the author of such law, "did always govern his own people by monarchy."
If Fuller revived the idea of natural law in modified form, JM Finnis revives it much more directly. In his Natural Law and Natural Rights (1980) he argues that there are "human goods"-such as acquiring knowledge, enjoying oneself, having friends-which are attained by exercising "practical reasonableness," the methods of which guide behaviour and therefore constitute the content of natural law. "Sound laws," he states, derive from "unchanging principles that have their force from reasonableness."
As with all philosophical debates, discussion of the nature of law immediately spills into discussion of closely related concepts. In this case the most important are rights and justice. Some of the greatest contributions of 20th century jurisprudence have been made in discussion of these fundamental concepts: the classics are, on the first, Ronald Dworkin's Taking Rights Seriously (revised ed. 1978), and on the second, CH Perelman's The Idea of Justice and the Problem of Argument (1963), John Rawls's A Theory of Justice (1971), and Robert Nozick's Anarchy, State and Utopia (1975). They are for another month.
