In August, 2011, I received a letter with a return address that read “Supreme Court of the United States.” There was no name on the envelope. I tore it open, wondering whether I was somehow being summoned for jury duty.

Inside was a two-page letter from John Paul Stevens. He had recently read my book “Contested Will,” about the controversy over the authorship of William Shakespeare’s plays. Like most scholars today, I freely acknowledged that Shakespeare had co-authored plays, and Stevens wondered if I might be open-minded on the subject of his having collaborated with the Earl of Oxford. Stevens also shared his doubts about Shakespeare’s literacy: the six surviving signatures of Shakespeare, affixed to legal documents, are each spelled differently. “Perhaps it is unfair to draw inferences based on a modern reader’s reaction to 16th century writing, but those examples have persuaded me that if he was the author, he must have had help in the preparation of his manuscript,” Stevens wrote.

Stevens, who died last month, at the age of ninety-nine, believed that Edward de Vere, the Earl of Oxford, wrote Shakespeare’s plays. As Tyler Foggatt recently recounted in this magazine, Stevens became interested in the controversy during a moot court, in 1987, called “The Authorship of Shakespeare on Trial.” Five years later, Stevens published an article, in the University of Pennsylvania Law Review, recasting in legal terms the case against Shakespeare, and largely sidestepping what has always been the Achilles’ heel of claims for Oxford’s authorship: dead men don’t write plays. Oxford was buried in 1604, but Shakespeare went on writing plays until 1614, many of which, including “Macbeth,” “Coriolanus,” and “The Tempest,” engaged contemporary events and sources. In 2009, Stevens accepted an award as “Oxfordian of the Year.”

Since this was likely to be my only chance to cross-examine a Supreme Court Justice, I wrote back to Stevens immediately. Over the next six months, we exchanged long letters. Stevens was keen on exploring weaknesses in the case for Shakespeare, and I was curious about what led so wise a jurist to embrace a conspiracy theory—and that’s the only word for it, since there’s not a shred of documentary evidence linking Oxford to Shakespeare’s plays, only speculation and surmise. To look back on my exchange with Stevens is a reminder of how firmly conspiracy thinking has taken hold in America, from anti-vaxxer propaganda to the belief that the moon landing was faked.

I explained at some length what was known about Shakespeare as a co-author, and how the culture of playwriting at the time precluded collaborations between professional dramatists and aristocrats like Oxford. It was not “impossible, but based on surviving documentary evidence, next to impossible” for Shakespeare to have co-authored plays with Oxford, I told him. I saw that Stevens’s secretary had both typed and signed his letter for him, so couldn’t resist adding, “As your own letter confirms, signatures, then as now, are problematic as evidence: I noticed that your letter to me is signed by someone else. It would be just as wrongheaded for me to conclude that you are illiterate and therefore need help signing your name.”

The Bible of the Oxfordian movement was J. Thomas Looney’s “Shakespeare Identified,” published in 1920. Stevens knew from having read my book that Looney, a member of the cultish Church of Humanity, had landed upon Oxford as an alternative candidate because the Earl’s life (inventively reimagined) dovetailed with Looney’s own nationalist and reactionary views. Looney’s interest in Shakespeare was more political than literary: he despised modernity and was profoundly anti-democratic. The plays of Shakespeare, understood as the works of an aristocrat, offered Looney a guide for a wished-for restoration of a repressive feudal regime, in which everyone knew his or her place.

In short, more was at stake than whether one writer or another had written the plays. Looney “despised the democratic values you have spent much of your life protecting,” I wrote to Stevens. I asked him, “How could someone so well versed in interpreting evidence reject the extensive documentary evidence linking William Shakespeare of Stratford to the plays in favor of what you yourself have called a conspiracy theory, confusing circumstantial evidence with documentary evidence?”

I went on to explain, perhaps too bluntly, why this mattered:

When one of the most revered legal minds and leaders of this nation works to legitimate one conspiracy theory, it makes it much easier for lesser minds to do the same with other conspiracy theories (and as a New Yorker, writing as the 10th anniversary of the destruction of the Twin Towers approaches, I am especially sensitive to this). A brief glance at our nation’s political landscape (in which, for example, so many believe that President Obama is not American, and even doubt the documentary evidence of his birth certificate) confirms that conspiracy thinking is not a neutral activity. Like it or not, your public expression of interest in the Oxford question has, to my mind at least, disturbing political implications.

I thought that my sharp challenge of a man more used to asking questions than answering them might mean the end of our exchange. But a week later another letter from him arrived. I opened this one more carefully. This time Stevens signed it. He ruled Looney’s beliefs inadmissible: