Lawyers lean heavily on the connected concepts of legal right and legal obligation. We say that someone has a legal right or duty, and we take that statement as a sound basis for making claims and demands, and for criticizing the acts of public officials. But our understanding of these concepts is remarkably fragile, and we fall into trouble when we try to say what legal rights and obligations are. We say glibly that whether someone has a legal obligation is determined by applying “the law” to the particular facts of his case, but this is not a helpful answer, because we have the same difficulties with the concept of law.