The municipal Jaw of England is divided into common Jaw and equity. This is so because in the middle ages, the judges of the courts of common law (the Court of Common Pleas and the Court of King's Bench) believed that they could not expand the existing law in order to solve new problems. They thought that they were bound by the established Jaw as found in their own earlier judicial opinions. Furthermore, they felt that it was the function of Parliament to change the law; therefore, it would be an unconstitutional usurpation of the legislative power for the courts to expand the law. Since Parliament in the middle ages was not an efficient instrument for law reform, the common Jaw began to stagnate.
W.H. Bryson, Virginia Law Reports and Records, 1776 - 1800, in Case Law in the Making : The Techniques and Methods of Judicial Records and Law Reports, 69-82, 53-84 (1997).