In McDonald v. Chicago, the Supreme Court will consider whether the Second Amendment right of armed self-defense found in District of Columbia v. Heller applies to states under the Fourteenth Amendment. Significantly, the question presented addresses not only the Due Process Clause, but also the Privileges or Immunities Clause, which has lain mostly dormant since the Slaughterhouse Cases in 1873. Whether armed self-defense is a privilege of citizens of the United States is, of course, an important question. But more fundamental is why, exactly, particular rights count as privileges of citizens of the United States. Many different answers might be used to strike down Chicago’s gun ban, but with very different implications for other cases. There are four main possible reasons armed self-defense might count as a privilege of citizens of the United States: such a right (a) is in the Bill of Rights, (b) was traditionally respected in 1868, (c) is generally respected today, or (d) is a natural right. Even if these four questions—“Is it in the Bill of Rights?” “Was it widely respected in 1868?” “Is it widely respected today?” and “Is it a natural right?”—all produce a “yes” for armed self-defense, they will certainly diverge in other cases, because not all traditional liberties are listed in the Bill of Rights, the American tradition of civil liberty has changed between 1868 and today, and neither the Bill of Rights nor our traditions track natural rights perfectly...