Economic theory distinguishes sharply between what a person wants and what he can have. "Preferences" describe what a person wants, and "constraints" describe the limits of what he can have. The collision of preferences and constraints yields the choices that economists study. The meaning of both terms is broad and flexible. Preferences and constraints help to distinguish between the internal and external viewpoints that H. L. A. Hart made famous. The internal viewpoint concerns preferences to perform legal obligations. A person who prefers to obey a law is willing to give up something to perform his legal obligation. The preference is intrinsic, not an instrument for securing something else of value. Conversely, a person who is indifferent to a legal obligation takes a purely instrumental approach towards obedience-he obeys only when doing so secures something else of value. What explains the distribution of preferences among people to obey a law? I will sketch part of the answer that emerges from economic and psychological studies. Finding an answer is important because when laws are reasonably just and many citizens intrinsically prefer to obey them, government is easier, and life is better than when most citizens are indifferent towards obeying the law.