The Seventh Circuit Court of Appeals recently decided McDonald v. City of Chicago, a challenge to Chicago’s gun ban. The case has major implications for protecting gun rights at the state level, but its importance goes further than that. Depending on what the Supreme Court does, it could make originalism — relying on the text of the Constitution and its amendments as they were understood when enacted — the accepted standard for interpreting the Bill of Rights, rather than the whims of a handful of justices.

The plaintiff’s case in McDonald is based on the Second Amendment, but also on the Fourteenth. Last year, in Heller v. District of Columbia, the Supreme Court ruled that the Second Amendment protects an individual’s right to own firearms against infringement by the federal government. But can a state or local government infringe that right? This question hinges on the constitutional principle of “incorporation” — the notion that the Fourteenth Amendment makes the states subject to the Bill of Rights.

When it was enacted in 1791, the Bill of Rights applied to the federal government only. Individual states could (and did) restrict free speech, for example, or have an established church. The states were beholden only to their own laws and constitutions and to certain provisions in Article I of the U.S. Constitution. After the Civil War, as Justice Clarence Thomas wrote in Zelman v. Simmons-Harris (2002), “the Fourteenth Amendment fundamentally restructured the relationship between individuals and the States and ensured that States would not deprive citizens of liberty without due process of law.” But what “liberty” was included in this guarantee, and what was meant by “due process”?

No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.

Ringing phrases, to be sure, but somewhat short on specifics. In fact, the meaning of every major clause in the first section is disputed, and has been since the amendment’s enactment in 1868.

Case law provides little clarity. The first major Supreme Court decision to grapple with these questions came in the Slaughterhouse cases of 1873, but — to quote Justice Thomas again, this time from his dissent in Saenz v. Roe (1999) — that decision “all but read the Privileges or Immunities Clause out of the Constitution.”  By adopting a narrow interpretation of the clause, the Court gave states wide latitude to enact laws they thought were necessary.

Towards the end of the 19th century, however, as various reform movements took hold, restrictive state laws came to be seen as an obstacle to progress.  So the justices eventually concluded, as Prof. James W. Ely of Vanderbilt Law School notes, “that the Fourteenth Amendment did confer a national standard of rights against the states.” But instead of reviving the “privileges or immunities” clause to enforce this standard (and thus reversing Slaughterhouse), they seized upon the “due process” clause.

That clause may simply seem to restate the Fifth Amendment requirement guaranteeing an individual’s right to a day in court and the protections of the legal process, this time applying it to the states as well as the federal government. That’s the way many modern originalists understand the clause. But as reform took hold, courts began interpreting it to mean that states had no power to deprive citizens of important rights, whether or not those rights were mentioned in the Constitution. The judges themselves would decide what rights fell under the clause’s protection. They called this doctrine “substantive due process” to distinguish it from the traditional day-in-court meaning, which came to be called “procedural due process.”

At first, substantive due process was used mostly to reverse state encroachments on economic choices, like freedom of contract. If a baker wanted to work 70 or 80 hours a week, no one could stop him. But as the doctrine evolved, justices used it to incorporate selective provisions of the Bill of Rights, making them enforceable against state governments (the Second Amendment, among other provisions, was excluded for various reasons).  And as the 20th century wore on, these two trends diverged: Economic regulation became popular among the progressives on the Court, while regulation of individuals’ “private” conduct became even less so. The justices modified the doctrine to fit their new preferences.

The result, Ely notes, was to create “an artificial division between economic rights, which the pro–New Deal court wanted to reject, and personal rights, which they wanted to expand to mean virtually anything.” Since the 1940s, the concept of substantive due process has been greatly expanded by the Court, not just to protect the Bill of Rights from abridgment by state action, but to create rights to privacy and abortion, a “wall of separation” between church and state, and many other inventions of the Warren and Berger Courts.