Judge Richard Posner of the Seventh U.S. Circuit Court of Appeals announced his retirement Friday afternoon, affording President Donald Trump the opportunity to replace a legal titan on the Chicago-based federal appeals court.

Posner is one of the most consequential legal figures of recent times, exerting significant influence on the practice and study of law from his perches on the Seventh Circuit and the University of Chicago Law School faculty. Something of an intellectual gadfly, he has written 50 books, 500 academic articles, and several thousand legal opinions on a wide range of subjects, a prodigious output surpassing even Supreme Court Justice Oliver Wendall Holmes, Posner’s hero and perhaps the most lettered Supreme Court justice of his time. And Posner is almost certainly the most read jurist of recent decades: The Journal of Legal Studies says he was the most cited legal scholar of the 20th century.

Posner subscribes to a method of judging called “pragmatism,” which seeks to balance the equities of each case and conform judicial rulings to the social, political, and economic arrangements of the times. He touted his commitment to pragmatism in announcing his retirement.

“I am proud to have promoted a pragmatic approach to judging during my time on the Court, and to have had the opportunity to apply my view that judicial opinions should be easy to understand and that judges should focus on the right and wrong in every case,” he said in a statement announcing his retirement.

His vaunted pragmatism is often difficult to define, oscillating between libertarian law and economic theories and pure utilitarianism. What’s more, there are few in the federal judiciary as comfortable discarding precedent as Posner.

“I don’t know what ‘existing law’ means except views currently held by many judges, lawyers, and politicians,” he wrote for Slate in August. “Those views are likely to be fluid, changeable — in accordance with new social needs, attitudes, and authority. Law means one thing to conservatives, another to liberals. It has no fixity.”

Posner has elsewhere suggested that federal judges do not need to study the Constitution.

As such, the problem of contradiction in his pragmatic approach was often compounded by unpredictability.

He is also known for his colorful style from the bench. He once included a picture of an ostrich with its head in the sand in a section of an opinion addressing a litigant who refused to acknowledge an adverse precedent.

Posner is also closely identified with love of cats. He compared himself to his own cat, Pixie, in an interview with The New Yorker.

” have exactly the same personality as my cat,” he told the magazine. “Cold, furtive, callous, snobbish, selfish, and playful, but with a streak of cruelty.”

President Reagan appointed Judge Posner to the bench in 1981. Posner will continue teaching, with a particular emphasis on “social justice reform.” His retirement took effect Saturday.

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