The philosophy of law concerns itself with questions about the nature of law and the concepts that structure the practice of law. Its topics will include the definition of law, or, if strict definition proves unfruitful, descriptions or models of law that throw light on difficult and marginal cases, such as international law, primitive law, and immoral or unjust law. Concepts that require understanding include those of a legal right or duty, of legal action and the place of concepts such as intention and responsibility, the nature of legal reasoning and adjudication, and the overwhelming political importance of the rule of law.
Considered strictly in empirical terms, law may appear to be a system of coercion: the salient fact about being under a legal duty to do X is that failure to comply will bring in its wake various penalties. Law seen this way is regarded as a sequence of predictions about what judges will do and what events will ensue upon performance of various kinds of action. In the writing of so-called legal realists, a term such as ‘right’ becomes one with which we describe to each other the prophecies we make of the behaviour of various officials. A difficulty with realism of this kind is that it offers no satisfactory account of the reasoning processes of those very officials, as they conceive themselves to be guided by a structure of rights and duties (similarly, if saying that a ball is out of court is a prediction of what the tennis umpire will do, the umpire cannot in turn cite the fact of the ball being out of court as the justification of his decision).
In normative terms, law is a system of rules that justifies demands for conformity, and the idea of legal duty is one of being bound by a rule, whether or not it is likely that suffering will follow upon failure to comply. Legal action, such as saying in the right circumstances words like ‘I do’ or ‘I hereby…’, creates a different kind of normative relationship between persons, such as marriage or contractual agreement, and the existence of such relationships is not, from the normative standpoint, the same thing as the likelihood of various consequences following various actions. In the same way a rule of grammar is not simply a prediction that various forms of words will or will not be understood.
The tradition following Bentham and John Austin, known as legal positivism, recognizes a sharp separation between law as it is, and law as it ought to be. Bentham criticized his predecessors, especially the 18th-century jurist Blackstone, for the ‘spirit of obsequious quietism’ visible in the assumption that actual law represented a God-given structure of rights and duties. In separating law as it is from law as it ought to be, Bentham saw himself as opening the way to utilitarian criticism of both the content and the ramshackle structure of English law. Legal positivism is sometimes taken to include other elements: the contention associated with Austin that laws are the commands of the sovereign; the contention that the study of legal concepts is to be distinguished from historical inquiries into the origins of laws or sociological enquiry into the effect of laws, or moral criticism of the workings of the law; the contention that law is a frozen, closed system (in German jurisprudence, a Begriffshimmel or concept-heaven) with a determinate logical structure, from which verdicts may be derived without reference to wider pragmatic, social, and moral issues; and even the view that moral judgements are themselves non-cognitive in nature, and for this reason no part of the essence of legal practice. Opposition to a picture that includes some or all of these elements will typically stress the inseparable connection between a legal system and the wider social and moral circumstances in which it is set. Judicial reasoning, for example, is not and perhaps should not be a self-contained formalistic application of determinate laws in defined ways, but is an exercise of practical reasoning that is, or ought to be, fully open to moral, political, and social claims. The issue of whether these points undermine the positivist distinction between law as it is and law as it ought to be remains open. Questions belonging not strictly to the philosophy of law but to political philosophy include those of the scope of law, the nature and justification of punishment, and the justification of the legislative and coercive power of the state. These wider concerns include assessing the Marxist critique of law as an instrument of oppression, necessarily reflecting the ideas and therefore the partisan interests of the ruling political class.