The U.S. Supreme Court last month struck down the Chevron doctrine, a 40-year-old precedent that had allowed federal regulatory agencies to expand their power by giving deference to their interpretations of vagaries in federal statutes.

The decision is a welcome blow to the federal regulatory state, and puts the work of divining congressional intent back to the judiciary where it belongs.

Writing for the majority, Chief Justice John Roberts noted that over the years the court often gave deference to executive agencies on questions of fact, when the agencies presented reasonable evidence, assuming that those working within the agencies had a superior understanding of the subject matter.

But, even as the regulatory bureaucracy became more encompassing and its output more complex, the courts never ceded to regulators their constitutional authority to rule on questions of law — to define the meaning of a statute where Congress has failed to provide clear language.

“The interpretation of the meaning of statutes, as applied to justiciable controversies,” remained “exclusively a judicial function.”

In 1946, Congress passed the Administrative Procedure Act to provide the basic guidelines for rulemaking and administrative adjudication, and judicial review of agency actions. In simple terms, the act gave the executive branch guidelines to make and administer rules and regulations as authorized by statute, and gave the courts the authority to review those actions within the bounds set by individual statutes.

But in 1984, in Chevron U.S.A. Inc. v. Natural Resources Defense Council Inc., the Supreme Court ruled that in the absence of clear language from Congress, courts were to give deference to agency interpretations — even if the court took a different meaning.

“The Court eventually decided that Chevron rested on ‘a presumption that Congress, when it left ambiguity in a statute meant for implementation by an agency, understood that the ambiguity would be resolved, first and foremost, by the agency, and desired the agency (rather than the courts) to possess whatever degree of discretion the ambiguity allows’.”

No greater boon could there have been to the administrative state. Congress is often vague, but even its “clear language” can be open to subjective interpretation. Regulatory agencies become power unto themselves when they are able to prescribe a law’s intent, write regulations tailored to that view, and then hold violators liable in their own administrative courts.

In essence, Chevron created a self-contained fourth branch that we cannot find in our reading of the Constitution.

While preceding rulings have minimized Chevron’s importance in appellate cases, it still has sway in district courts. The doctrine has influenced some 18,000 federal court decisions, according to research cited by Justice Elena Kagen in her dissenting opinion.

No more.

The majority held that the Administrative Procedure Act is clear: where there is judging to be done, it will be done by judges.

It is a forlorn hope that Congress would be clear and thorough in its lawmaking. But under no circumstances should regulatory agencies be allowed to assume the prerogatives of all three branches.