In The Problematics of Moral and Legal Theory, Judge Richard Posner asks a critical question: can moral philosophy ever help the practice of law? Posner answers this question with a resounding no.
Take a simple example. Suppose that a robber is shot dead at the scene of the crime and, the shooter is charged with murder. Should he be convicted? A pacifist might say yes, since killing is always wrong. Others would say no, since he acted in self defense (killing is not always wrong). Others would say that it depends on whether he was threatened.
If you were the judge or the jury, who should you believe? Posner argues that you will believe the person who most closely approximates your preconceived beliefs. In other words, a pacifist prosecutor will have a hard time convincing anyone who believes in self-defense, since each and every moral philosophy can be countered by another moral philosophy.
Posner goes on to argue that moral philosophy can help us with things that we all agree on, like "democracy is good" and "freedom should be protected." The problem is that no case before a court ever deals with such broad disputes. Instead, each side lines up its moral arguments, and the judge basically ignores them all. Posner also shows that moral philosophy is of some use in changing what people agree is right, as with Martin Luther King, Jr.
In the down and dirty disputes before a judge, though, Posner says that we have to rely more on our gut reaction, economics, and sociology than moral theory. The judge must ask, "If I did X, would society be better off?" Posner calls this legal pragmatism, the hope that the law can be rationalized along empirical grounds. He is careful to distinguish this stance from philosophical pragmatism and moral relativism. The former worries only about ends, while Posner explicitly worries about means, and the latter would not allow anyone to ever say that "murder is wrong." Posner has no problem with moral precepts that everyone agrees with.
The downside of this book is that it is way too long. The first few chapters outline most of his argument, and the rest of the book deals with legal history and particular examples from supreme court cases. Law students might find these parts worthwhile. Those who are interested in philosophy and law should read this book, as should those who want a look at how judges think and work. Whether you agree with him or not, his exploration of the topic is cool and complete.