Since the Enlightenment, the project of building human life based on reason rather than transcendence has resulted in radical changes in the western world. Legal philosophers like John Austin sought to describe the legal system as a social fact, rather than as an expression of Natural law. Austin's system revolved around the idea of law as command, and was very simple. A law is a command of the sovereign, backed by threat of force. Austin also wrote on the seperation thesis - the idea that law and morality are separated, as opposed to Aquinas' overlap thesis.
A few problems appear here: who or what is the sovereign, and is the threat of force the real source of the law's power? Can the sovereign make any law it wishes, and is the law still binding? What is the law's relationship to morality?
Austin's brand of legal positivism was the standard in jurisprudence for several decades; then the Holocaust took place. The Nazi's use of the German legal system made it clear that the law was a much more complicated thing than a simple command backed by force. The imfamous "I was just following orders" defense was a major issue for legal philosophers.
As far as I understand, there have been three prominant perspectives on the law in the 20th century. The first to develop was legal realism - basically a group of really cynical judges. Remember, Aquinas was a theologian and Austin was a lawyer; the legal realists tended to be judges and so they had a different perspective.
The realists thought the idea of a theory of law was silly; better to actually watch the law in practice. Their answer as to the nature of law: whatever the hell a judge says it is. They don't start with rules and facts, they begin with a hunch, and work backwords from there. They argue this isn't a deficiency: this is the way the law is, and it is actually a good thing. It is the only way possible to achieve equity.
As far as I can see, this is a pretty court summary of how the courts work. Supreme court judges will trash cases brought before them - then agree with the conclusion of the case, and simply rework it themselves. They start with their conclusion and work backwards. Appeals are not about attacking conclusions per se, but rather about attacking the conclusion for an appeal.
And boy oh boy, this seems to be about right. I'm sure Joel will be happy to attest to the seeming randomness of some court decisions. When I was writing an essay on Creationism and Evolution in the American courts, the court decisions really seemed like the whim of whatever particular judge was presiding.
The legal realists are too wishy washy for some, of course. Next time I'll talk about the responses to this, hopefully in a single post.