Saad Gul is a Senior Litigation Counsel at Washington Legal Foundation.

Picture this: The law that covers your business, your contracts, and your responsibilities is changed overnight. No vote. No hearing. No opinion from your courts. Just a decision by a federal judge far away. He has never walked your streets or smelled your smokehouse. He does not know your traditions or your hometowns. Still, you are bound. The cause? A judicial device called the Erie guess.

The Erie guess is a direct consequence of the foundational Erie doctrine, articulated in Erie R.R. v. Tompkins, 304 U.S. 64 (1938). The Erie doctrine remains a cornerstone of American federalism. This doctrine provides a clear directive: A federal court, hearing a diversity case, must apply the substantive law of the state in which it is situated. Federal procedure applies. But state authority defines core legal rights. This ensures consistency across state and federal courts.

Yet this principle often falters when state law offers no clear answer. The Erie guess is what happens when a federal court, bound to apply state law in a case, ventures to predict how the state’s highest tribunal might resolve a question it has never confronted. The court’s prediction becomes binding law. The original goal was fidelity to state law. Yet, through repeated Erie guesses, federal courts have ended up making state law themselves.

But judges are not supposed to replace the law. They are supposed to apply it. Still, when states don’t speak, Erie guessing tempts federal courts to fill the void. That speculation becomes binding. Interpretation becomes legislation. It is like filling out your neighbor’s will, stamping it “approved,” and calling it enforceable. The form is present. The authority is not.

That absence of authority ought to compel restraint. Instead, it invites speculation. Federal judges often begin with care in addressing gaps in state law. They look at old rulings, commentary, and legislative signals. They fill the silence with what they believe the law might be.

But that belief hardens. A guess made today becomes doctrine tomorrow. Each Erie guess builds on the one before it. The guesses compound. Before long, a judge ruling from a distant bench is creating rules that apply like statutes. That is not state law as the Constitution envisions it.

This is no academic exercise. It is happening in actual courtrooms. It affects real people, with real cases, and real consequences.

This abstract problem became concrete in two recent Uber cases. The courts were dealing in tragedy, not abstract legal theory. In the first Uber case, criminals masquerading as passengers killed a man doing honest work driving an Uber car. In the second, a predator impersonated a driver and assaulted a woman seeking safe passage.

At the heart of both cases was a single question: did California or Washington law impose a special duty on Uber? This was not an academic puzzle. It was tort law in its rawest form. Federal courts had to determine the very existence and scope of a specific legal duty in the face of brutal facts. The question was plain: does your law, California, or yours, Washington, impose such a duty on this platform? Both the California and Washington Supreme Courts were asked. Both declined to answer.

The Ninth Circuit moved forward anyway. The court anticipated how the state courts might rule and proceeded as though that prediction was settled law. In doing so, it created a new legal duty from its own prediction. Olympia was silent. Sacramento said nothing. Yet a San Francisco federal court imposed a new legal obligation affecting millions of citizens. It did so despite the deliberate silence of two state legislatures and two state supreme courts. That is the Erie guess: lawmaking by judges with life tenure, no constituents, and no accountability to those who must live under the rule they imposed.

This is not judging in any recognizable form. It is policymaking or legislation by robe. State law gives way to federal guessing. A gap becomes a command. No legislature passed it. No citizen asked for it. Yet the ruling still binds Californians and Washingtonians.

But the Constitution reserves lawmaking to those close to and accountable to the people. So state law is for the state courts and the state legislature. Federal courts may not patch over gaps with Erie guesses. The stakes for federalism were clear. That’s why WLF urged the Court to reaffirm Erie’s limits in Uber v. Drammeh in a May 14, 2025 amicus brief supporting Uber’s petition for certiorari.

WLF urged adherence to Erie: follow state law where it speaks, and keep the ink dry where it does not. No Erie guesses. Federal courts have no business ghostwriting state codes. No answer from the state? No answer from the federal court. That is Erie. That is the law.