Substantive due process is the theory that the Supreme Court was assigned not one but two roles by the US Constitution, one clearly explained by the framers, namely the job of interpreting the meaning and proper application of laws, and the other which they merely "hinted at" by the due process clause in a manner so vague that it took until 1900 for anyone to formally recognize its existence, namely the job of striking down "bad laws" which "although possibly consistent with existing US laws, violate fundamental principles of fairness and justice as interpreted by the nine justices". According to this view, a law may be unconstitutional, not because it violates anything in the Constitution, but because it is inconsistent with its general theme of liberty and justice. In practice, this theory amounts to a prescription for judicial activism, taking the unrelated due process clause as “legal basis” for rulings based solely on justices’ own views of what is fair and just. This could not have been intended by the framers, or indeed anyone with the least bit of concern for objectivity in law.
The due process clause appears twice in the Constitution, first in the Fifth Amendment, which reads ” No person shall ... be deprived of life, liberty, or property, without due process of law”, and again in the Fourteenth Amendment, which applies this same idea to states.
The goal of these amendments was to establish the United States as a nation in which the rule of law is strictly adhered to. This means, firstly, that the government must follow its own laws, and secondly, that any action it takes which deprives a citizen of any of the three rights mentioned by the amendments is illegitimate unless there is legal basis for it. In other words, the government exists for the implementation of law, and may only act in that capacity. These amendments were not intended to place additional restrictions on what laws may be considered lawful, but only to give citizens assurance that laws are to be the guiding force behind all governmental action. In other words, even in the absence of laws forbidding the government from taking some action, the action is still off limits to the government if it would deprive a citizen of life, liberty, or property.
Advocates of substantive due process want to interpret the amendment to mean, not that the government is restricted in what it may do even in the absence of any laws restricting it, but that the laws themselves must meet some basic condition of “fairness” implicit in the phrase “due process of law”. One interpretation deals with what should happen in the absence of law, the other deals with what should happen in the presence of “bad laws”.
There is simply no justification for the latter interpretation. The phrase “due process of law” comes from the Magna Carta, which strongly influenced the US Constitution, where it is employed in the following sense:
"No man of what state or condition he be, shall be put out of his lands or tenements nor taken, nor disinherited, nor put to death, without he be brought to answer by due process of law."
This is simply saying that the government may not prosecute or harass citizens arbitrarily i.e., without going through the proper legal procedure. It relates to the narrow issue of criminal prosecution. It hasn’t anything to say about what kinds of laws may be passed. Indeed, how many laws call for putting specific individuals to death, or for taking their lands? If the government were to take such actions, it would be based on some more general law, or no law at all. The Magna Carta says that only in the first case can the action be legally permissible.
Even if we ignore the Magna Carta and simply try to interpret the phrase “due process of law”, what is it actually saying? Just that there are laws and a legal process associated with forming and applying them, as well as laws which determine what may be a law, which the government must abide by. The doctrine of substantive due process turns this principle on its head by introducing unlimited subjectivity into the realm of law. It is impossible to have anything like “rule of law” if the Supreme Court is able to carry on the task of distinguishing between constitutionality and unconstitutionality without even looking at the Constitution, but merely by “looking into their hearts” and asking whether a given law fits their own conception of justice. Indeed, there is hardly an issue dividing democrats and republicans that does not reflect disagreement over the proper interpretation of justice. For there to be a real distinction between “interpreter of the law” and “lawmaker”, the former cannot be given something to interpret as vague as “justice”. And nothing in the Constitution says they are.