Legal positivism is a doctrine about the nature of law according to which (a) all laws are laid down or posited by a certain person or procedure, and (b) something counts as a valid law of a certain system in virtue of being laid down by a certain someone or according to a certain procedure. In other words, the legal validity of a rule or decision depends on its sources (e.g. where it has come from, and how, and when), rather than its merits (e.g. whether or not it is a good rule or decision). This way of understanding law was made famous during the nineteenth century by the ‘command’ theories of law espoused by Jeremy Bentham and John Austin. According to these theories, something is law if it has been commanded by a Sovereign, and is backed up by the threat of a sanction in case of non–compliance.
Command theories help us to understand the posited nature of law, allow us to identify and understand what law is before considering whether it is morally good or bad, and foreground the role which coercion plays in the law, and so furnishes us with a legal theory which attempts to tell citizens subject to the law exactly the sort of thing they are dealing with.
Unfortunately, however, the command theory makes it difficult to understand how legal systems work as a system. Each law is a law because it is posited by an act of the Sovereign, and so each law appears to be self–contained and self–sufficient, unified only in that all laws have in fact been commanded by the present Sovereign in the way explained by the command theory. This, however, fails to explain the way in which legal systems seem to have a life of their own, independently of the lives of the Sovereigns, or legislatures, which posit their laws. Legal systems remain in force, and are capable of altering the laws which comprise them, and of creating new laws, across time, and they retain these characteristics even when one Sovereign dies or one legislature dissolves and a new one ascends to the throne or is reconvened. Twentieth century legal theorists H.L.A. Hart and Hans Kelsen both criticised these weaknesses in Austin’s command theory of law, and, in their own separate ways, set out to explain what it is that unifies laws into legal systems, and which allows legal systems to regulate their own creation: to determine which laws belong to the system, alter existing laws, and make new ones, according to their own internal procedures for so doing.
How can legal systems pull off this amazing trick of regulating their own composition and creation?