Reacting to my explanation of substantive due process, Jonathan Rowe makes the point that the primary substantive protections in the Fourteenth Amendment were supposed to be in the privileges or immunities clause, not the due process clause, and that it was only the evisceration of the privileges or immunities clause in Slaughterhouse that led to the concept of substantive due process.
This is, in large part, correct. But the concept of substantive due process, as I explained, predates even the ratification of the Fourteenth Amendment. The fact is that the ideas of “procedure” and “substance” are not as distinct as it seems at first. The concept of substantive due process reflects the fact that you cannot assess a law’s legitimacy on the procedural aspects alone. As Justice Field—one of the greatest of substantive due process theorists—put it,
The Constitution deals with substance, not shadows. Its inhibition was levelled at the thing, not the name. It intended that the rights of the citizen should be secure against deprivation for past conduct by legislative enactment, under any form, however disguised. If the inhibition can be evaded by the form of the enactment, its insertion in the fundamental law was a vain and futile proceeding.
When the government deprives a citizen of his liberty, in the pursuit of something it has no authority to do (i.e., by an enactment which is not legitimate law), then the government is necessarily depriving him of liberty without giving him due process of law. (This would, of course, overlap the privileges or immunities clause properly understood, and it might also overlap the equal protection clause. But that doesn’t mean that a proper understanding of the privileges or immunities clause would entire deflate the concept of substantive due process, which is a perfectly legitimate legal concept.) And to discover whether a law does something illegitimate in disguise, requires the court to look at the substance of the legislation. Otherwise a law which pretended to just be a health and safety law could, in reality be a disguised attempt to harm an unpopular minority.
Incidentally, I’m quite attracted to the theory (advanced, if I recall correctly, by Akhil Reed Amar, but I’m not sure) that the three clauses of the Fourteenth Amendment are aimed at the different branches of government: the privileges or immunities clause regulates the legislature, the due process clause regulates the judiciary, and the equal protection clause regulates the executive.