In late 19th century America, the federal Supreme Court declared unconstitutional many laws that restrained liberties. This article considers the legal thought that characterized what came to be known as the Lochner era. Distinguished scholars of that time developed a theory of an unwritten constitution based upon a concept of law as custom. The theory considered the constitution to be an instrument of customary and unwritten law.This approach came out of a long tradition in Anglo-American law. In 17th century England, common law lawyers such as Sir Edward Coke regarded law as custom. Their philosophy is now known as "classical common law theory." It saw law by definition as unwritten and immemorial custom, based on reason and the consent of the people. The exponents of this approach applauded common law and distrusted statutes and any prerogative of the king.Classical common law theory was imported to the American colonies and developed there from the revolution through the 19th century. Legal scholars in the Lochner era later relied on this tradition in justifying active judicial review.