So, if we didn't have substantive due process, what would we have? Well, we would have procedural due process. And Justice Story in 1833 gave a description of the Due Process Clause. 1862, there's a huge debate in Congress over due process objections to the second Confiscation Act. Lots and lots of congressmen say, "We think Justice Story's definition of due process was correct."
Well, what did Justice Story say? It said, "The other part of the clause," talking about the Fifth Amendment, "is but an enlargement of the language of Magna Carta." He quotes it in Latin. "Neither will he pass upon him or condemn him but by the lawful judgment of his peers or by the law of the land. Lord Cooke says that these latter words per legem terrae, by the law of the land, mean by due process of law, that is without due presentment or indictment and being brought in to answer thereto by due process of the common law, so that this clause, in effect, affirms the right of trial, according to the process and proceedings of the common law."
So, that phrase, "the process and proceedings of the common law" gets quoted over and over again in 1862. If you're trying to understand what is the immediate background to the Fourteenth Amendment, what does due process of law mean? It means judicial process, the process and proceedings of the common law.
States are not allowed to take away people's life, liberty, or property unless they go through a judicial process, according to the process and proceedings of the common law. It's not a guarantee that prevents states from designating particular people as part of a ius publicum, as opposed to the ius privatum. It's merely a procedural guarantee.
It does have aspects protecting vested rights, saying a legislature can't just confiscate. A legislature, in order to get somebody's property, has to say that, if somebody does something, "We'll go through a judicial process to get it." But aside from that sort of vested rights, "substantive" due process... Aside from that sort of thing, the Due Process Clause is procedural.