Sources of law
In the United States, the law is derived from four sources. These four sources are constitutional law, administrative law, statutory law, and the common law (which includes case law). The most important source of law is the United States Constitution. All other law falls under, and is subordinate to, that document. No law may contradict the Constitution. For example, if Congress passes a statute that conflicts with the Constitution, the Supreme Court may find that law unconstitutional. Notably, a statute does not disappear automatically merely because it has been found unconstitutional; it must be deleted by a subsequent statute. Many federal and state statutes have remained on the books for decades after they were ruled to be unconstitutional. However, under the principle of stare decisis, no sensible lower court will enforce an unconstitutional statute, and any court that does so will be reversed by the Supreme Court.
Also, certain practices traditionally allowed under English common law were outlawed by the Constitution, such as bills of attainder and general search warrants.
American common law
The United States and most Commonwealth countries are heirs to the common law legal tradition of English law; for example, U.S. courts have inherited the principle of stare decisis. A small number of important British statutes in effect at the time of the Revolution have been independently enacted in nearly identical form by U.S. states. Two examples that many lawyers will recognize are the Statute of Frauds and the Statute of 13 Elizabeth. Such English statutes are still regularly cited in contemporary legal writings about their modern American descendants. Occasionally, contemporary U.S. courts may cite a pre-Revolution case when discussing the evolution of an ancient judge-made common law principle into its modern form, such as the heightened duty of care traditionally imposed upon common carriers.
Although the courts of the various Commonwealth nations are often influenced by each other's rulings, American courts rarely follow post-Revolution Commonwealth rulings unless there is no American ruling on point, the facts and law at issue are nearly identical, and the reasoning is strongly persuasive. The earliest American cases, even after the Revolution, often did cite contemporary English cases. This was because appellate decisions from many American courts were not regularly reported until the mid-19th century; lawyers and judges, as creatures of habit, used English legal materials to fill the gap. But citations to English decisions gradually disappeared during the 19th century as American courts developed their own principles to resolve the legal problems of the American people. The number of published volumes of American reports soared from eighteen in 1810 to over 8,000 by 1910. Today, the vast majority of American legal citations are to domestic cases. Sometimes, courts, and casebook editors, do make exceptions for opinions on issues of first impression by brilliant British jurists, like William Blackstone or Lord Denning.
Some adherents of originalism and strict constructionism such as Justice Antonin Scalia of the United States Supreme Court argue that American courts should never look for guidance to post-Revolution cases from legal systems outside of the United States, regardless of whether the reasoning is persuasive, with the sole exception of cases interpreting international treaties to which the United States is a signatory. This position follows inevitably from the philosophy of originalism, which posits not only that the Constitution is the ultimate source of judicial authority in the U.S., but that the only proper analysis of the document consists of discerning the document's original meaning at the time of its adoption. Therefore, discussion of British law that post-dated the Constitution is irrelevant as it sheds no light on the original meaning of the Constitution. Others, such as Justices Anthony Kennedy and Stephen Breyer, disagree, and cite foreign law from time to time, where they believe it is informative, persuasive, useful or helpful. However, foreign law has never been cited as binding precedent, but merely as a reflection of the shared values of Anglo-American civilization or even Western civilization in general.