The independence of the different departments of government is especially emphasized with reference to the functions of the judiciary. It is a fundamental principle of constitutional government that the courts, which are organized for the purpose of deciding controversies, shall be free from bias or extraneous influence, and that judicial questions shall be determined by the courts, and not by the executive or the legislature. Therefore it is fully recognized that under our system of government questions which are in their nature essentially and necessarily judicial cannot be passed upon save by judicial tribunals; and the decisions of such tribunals are final and cannot be reviewed by any other department of the government. Neither the legislature nor the executive can, as between adverse claimants to property, vest the title in the one or the other, and thus in effect adjudicate the ownership of the property; nor can the legislature determine that an individual has been guilty of crime, and subject him to punishment therefor without a judicial trial. These principles are involved in the usual constitutional guaranties of due process of law and prohibition of bills of attainder; but a fuller exposition of them will be given in another place. (See below, §§ 228, 258.) However, an important application of these principles should be noticed here, involving the power of the legislature to interfere with the judgments of the courts. It is well settled that when a judgment has been rendered, the legislature cannot by a statutory enactment undo what the courts have done, or reverse their decisions. For instance, although the legislature may authorize new trials, it cannot provide for a new trial in a case already tried; and although it may provide for appeals, it cannot of its own authority in a particular case review or reverse a decision of a court.

Some judicial authority is exercised in England by the House of Lords, but in so acting it is a court, and not a legislative body. Likewise at one time in New York the senate was a court for the correction of errors, to which appeals might be taken from the courts of the state, but this also was the exercise of judicial, and not legislative, functions. Legislative divorces have been recognized as valid in some of the states, it being considered that the power to grant a divorce was not a judicial power; but in many of the states, there are now constitutional prohibitions of legislation with reference to granting divorces in specific cases, although, of course, the legislature may provide the laws in accordance with which divorces may be granted by the courts.