The case involving the Deferred Action for Childhood Arrivals program, or DACA, is a prime example. When the case, Department of Homeland Security v. Regents of the University of California, was argued last week, no one disputed that President Trump can rescind the deportation deferral that President Barack Obama granted to hundreds of thousands of undocumented young people who were brought to this country as children and who have made their lives here. The case is not about DACA’s legality or presidential discretion. It’s about rules: whether in rescinding DACA, the president adhered to the core principles of administrative law. Judges in four federal judicial districts found that he did not.

“Administrative law” may sound like a boring subject, but it isn’t. It’s both fascinating and, in the 73-year history of the Administrative Procedure Act, more important than ever for providing the legal structure in which encounters between citizens and their government take place. The Administrative Procedure Act doesn’t tell the government what to do. It simply requires that actions of federal agencies be supported by reasoned decision-making. When challenged, agencies have to provide explanations that are plausible and consistent rather than “arbitrary and capricious.”

It was on this basis that the administration’s effort to add a citizenship question to the 2020 census crashed and burned in June. The judges who blocked the plan, including Chief Justice John Roberts in his majority opinion, did not, as Attorney General Barr implied in a speech last week to the Federalist Society, “inquire into the subjective motivation behind governmental action.” In fact, they rather ostentatiously steered clear of speculating about motive, although the rest of us were free to see “vote suppression” written all over the plan. The judges simply put the administration’s stated reason (to aid the Justice Department’s enforcement of the Voting Rights Act) side by side with the facts (the request had not originated with the voting rights enforcers) and decided that the distance between the two was too great to meet the test of reasoned decision-making.

In similar albeit less consequential fashion, finding a lack of reasoned decision-making was behind a federal appeals court’s recent invalidation of a 5-cent increase in the price of a first-class stamp. I chose that case to write about in a recent column as a way of depicting administrative law in action, divorced from any political valence. The opinion for the unanimous three-judge panel of the federal appeals court in Washington was written by Judge Neomi Rao, a conservative administrative law specialist named by President Trump, and joined by an Obama-administration appointee, Judge Patricia Millett.

The president would almost certainly have gotten away with rescinding the DACA program if he and his lawyers had simply said, “We don’t like DACA, it’s inconsistent with our approach to immigration policy in the following ways, and we’re getting rid of it.” But the lawyers couldn’t say that because President Trump had promised “the Dreamers” at the start of his administration that they would have his continued support. So the explanation the lawyers offered was that they had to terminate the program because they had discovered that it was illegal.