Herman Gundy stands convicted of violating a law that, for all intents and purposes, doesn’t exist. You may recall from high school civics that the Constitution separates the powers of the federal government among three coordinate branches. You may also recall from “Schoolhouse Rock” that a bill becomes a law after it’s passed by the two houses of the legislative branch and signed by the president. Unfortunately for Gundy, things are no longer so straightforward.

The Sex Offender Registration and Notification Act (SORNA) set up a national system of sex offender registration and made it a crime for sex offenders to fail to register with local authorities when they moved to a new state. While serving time on a federal drug charge, Gundy was transferred from prison in Pennsylvania to a halfway house in Brooklyn. According to the government, that counted as interstate travel sufficient to trigger reporting obligations of which he was never advised.

Gundy’s appeal of the conviction, to be heard by the Supreme Court this fall, addresses an odd facet of SORNA: while Congress laid out in detail those persons who would be required to register in the future, it did not determine who would have to register if the conviction occurred before SORNA was passed in 2006. Congress delegated that question to the Attorney General, and gave no guidance on how the determination should be made. Gundy’s sex offense is among those that predate SORNA, and therefore he was convicted of failing to register not based on anything Congress wrote in any law, but based on an administrative regulation written by the Attorney General.

Assigning such a determination to the executive branch raises a long-dormant canon of constitutional interpretation, the nondelegation doctrine. The basic idea is simple: the Constitution vests legislative power in the legislative branch and the legislature can’t delegate the power to write laws to a different branch. It’s a principle recognized by Chief Justice John Marshall in the first decades of the republic—indeed, its roots can be found in enlightenment thinkers such as Locke and Montesquieu—and reaffirmed in many Supreme Court opinions. The separation of powers means, at the very least, that the powers must remain separate.

But despite the Court often affirming the importance of the nondelegation doctrine in the abstract, the justices have disapproved of delegations in only two cases, both decided in 1935. While the doctrine is purportedly alive, many now treat it as the Black Knight of constitutional law, forever asserting “I’m not dead, yet.”

Perhaps this time is different. The Court’s cases say that delegations can be approved as long as Congress provides an “intelligible principle” to guide the delegated discretion, but here there is no principle, intelligible or otherwise. The statute empowers the executive branch to do as it likes, with no standards to follow. This case, therefore, presents an excellent opportunity for the Court to protect the separation of powers in an area where it has otherwise been skittish.

The doctrine of separation of powers is not some mere appeal to procedural formality, but a guarantee of our rights as citizens. The English jurist William Blackstone defined tyranny as the vesting in a single body of “the right both of making and of enforcing the laws…wherever these two powers are united together, there can be no public liberty.” It is this tyranny the Constitution was partially designed to prevent. The Cato Institute, joined by the Cause of Action Institute, has filed a brief supporting Gundy, arguing the Court should overturn his conviction and ensure that, as John Adams put it in his draft of the Massachusetts Constitution, “[t]he executive shall never exercise the legislative and judicial powers…to the end it may be a government of laws and not of men.”