Perhaps the most marked distinction between the governmental system of Great Britain and that of the United States, is that in Great Britain the courts cannot question the validity of an act of Parliament regularly passed on the ground that it is in violation of the constitution, while in the United States such power is regularly and frequently exercised by the courts with reference to state or federal statutes. The reasoning on which this exercise of power by the courts in the United States is based is the following: The law-making power of the federal or of a state government exercises only delegated authority, and it cannot transcend constitutional limitations imposed upon it; and therefore its acts, when without authority or in violation of constitutional limitation, are invalid. If such a statute were to be regarded by the courts as a part of the law, binding upon them, then constitutional limitations, transgressed by such a statute, would be of no validity whatever. Consequently when in any case before a court it becomes necessary to determine what the law is, and a statute is relied upon as being an authoritative statement of the law, the court must decide whether or not the statute is valid. This reasoning applies with equal force to any action of the executive department which is without authority or in violation of constitutional restrictions. This peculiar characteristic of our form of government, which results in giving to the courts a much more important function than any exercised by the courts in other countries, results, then (1) from the conception of ultimate sovereignty as residing in the people and not in the government; (2) from the conception of the government as exercising only delegated and limited powers; and (3) from the division of the powers of government among three separate departments, each of which has authority only so far as the powers of government provided for in the constitution are conferred upon it. The courts are required under our form of government to exercise such power in order to arrive at a proper basis for deciding cases before them.

It is to be observed that this function of the courts is not the primary or the principal purpose of their creation and recognition as a department of government. Courts are created primarily to decide legal controversies; but in deciding such controversies it is necessary for them to determine what is the law as applicable to the particular case, and as incidental to the exercise of this function they may have to decide whether a statute or an executive act relied upon by one party or the other is valid, or whether, on the other hand, it is invalid because in excess of the power conferred upon the department which has attempted to act, or is in violation of some constitutional provision or limitation. It is a function of the lawmaking power to determine prospectively what shall be the law, and to express that determination by adding to or modifying or repealing the existing law by statutes taking effect from the time of their enactment. (As to ex post facto and other retrospective laws, see below, §§ 59, 272.) The courts, on the other hand, decide cases submitted to them with reference to what the law was at the time the controversies to be determined arose, by which the rights of parties to such controversies are to be adjudged. Their principal concern is as to what is, or rather what has been, the law up to the time of the decision, not what shall be the law for future cases.

It is true that, having decided what the law is, a court will be likely in future cases to adhere to the views expressed in previous decisions, and the desirability of having the rules of law on which persons may act and rely stable and settled will incline the courts to adhere to their former decisions, which will be regarded as precedents in subsequent cases. But the act of the court in determining what the law is in a given case is not primarily for the purpose of ascertaining it for future cases, but in order that the case before it may be rightly decided. It is erroneous, therefore, to speak generally of the judicial department as having power to interpret or declare the law as though it were especially created for the purpose of interpreting the constitution and the acts of the other departments of the government in order that the people shall be advised as to what they mean. It is assumed rather that the constitution and the law, both written and unwritten, are known, and that persons whose controversies come before the courts have acted with reference to the law as it existed and are bound to knowledge thereof; and the courts, therefore, on that basis determine only retrospectively what law applies in the settlement of controversies which have already arisen. Nevertheless, as a result of such determination, persons may in the future be guided and greatly assisted in following the law, by having reference to what has already been decided in previous cases.

The preceding considerations lead to some other important conclusions as to the proper province of the courts, in passing upon the constitutionality of statutes. First, it will always be borne in mind by a court that the legislative department, on whose authority the statute rests, is a co-ordinate branch with the judicial; that there is no superiority as between them; that each is vested with power and discretion within the scope prescribed for it by the constitution; and therefore that an act of the legislative department is entitled to every presumption in its favor, and that to question its validity is the exercise of a very delicate and extraordinary power, to be resorted to only in the last extremity and when the rights of the parties to the litigation are found necessarily to depend upon its construction. The courts then, instead of being zealous to interpret and determine the validity of statutes with respect to their constitutionality, in order that the people may know and act accordingly, will discharge such duty with great reluctance and reserve, the importance of making the law clear for future cases being subordinate to that of leaving the legislative department free in the exercise of its constitutional prerogative of law making. Moreover, the courts will entertain every presumption in favor of the validity of a statute called in question, and declare it to be invalid on constitutional grounds only where it is plainly and clearly in conflict with the constitution. They will not pass upon such a question save in a case in which it is necessary to do so in order to adjudicate the real and substantial rights of the parties in that case; and they will avoid, if practicable, considering such a question except after full argument and a consultation in which all the judges of the court are present.