Had the definition that emerged from such discussions been used solely to help data collectors, it would, Stern believes, have served a necessary and constructive purpose. The problem arose when groups started employing it as a hate-speech code instead. In our conversations, Stern described this development as an “abuse,” saying that he wished, in retrospect, that “guardrails” had been installed to prevent it. “None of us anticipated that it would be used as this blunt instrument to suppress pro-Palestinian speech,” he said.

Still, as Stern concedes, the I.H.R.A. definition was not an academic exercise. It was created to help governments and civil-society organizations identify incidents of antisemitism. It’s not surprising, then, that some proceeded to use it to address the problem, often through legal means. In the view of some critics, the real problem is not the definition’s faulty application but its flawed content, in particular its vague examples of anti-Israel speech, which blur the line between antisemitism and anti-Zionism. In 2021, an alternative definition called the Jerusalem Declaration on Antisemitism sought to draw finer distinctions. In the preamble, it stated, “Hostility to Israel could be an expression of antisemitic animus, or it could be a reaction to a human rights violation, or . . . the emotion that a Palestinian person feels on account of their experience at the hands of the State.” The Declaration went on to list some examples that were antisemitic, such as “holding Jews collectively responsible for Israel’s conduct,” and others that were not. “Criticism that some may see as excessive or contentious, or as reflecting a ‘double standard,’ is not, in and of itself, antisemitic,” it affirmed. “Opposing Zionism as a form of nationalism” was also not, on its face, antisemitic.

The Jerusalem Declaration on Antisemitism has been signed by three hundred and fifty scholars, including the historian Omer Bartov and Susannah Heschel, the chair of the Jewish-studies program at Dartmouth. Stern has not signed it, nor has he formally repudiated the I.H.R.A. definition. Yet to listen to him speak these days is to wonder which definition he considers a more fruitful guide. One evening in January, I attended a talk he delivered at the West End Temple Sinai Congregation, in Neponsit, Queens. Stern was dressed in chinos and an oversized wool blazer; Margie, who served as the congregation’s rabbi for fifteen years, until 2020, watched from the audience.

The topic of Stern’s talk was “Antisemitism and Anti-Zionism.” A few weeks earlier, the U.S. House of Representatives had adopted a resolution affirming that these two things were indistinguishable. Since October 7th, the leaders of many Jewish organizations have done the same, seizing on the rhetoric of groups like Students for Justice in Palestine, whose national steering committee distributed a tool kit that described the Hamas attack as a “historic win for Palestinian resistance.” In November, Jonathan Greenblatt, the director of the Anti-Defamation League, proclaimed that the “tsunami of anti-Jewish hate” on America’s campuses “clarified and confirmed that fanatical anti-Zionism from the hard left is as dangerous to the Jewish community as rabid white supremacy from the extreme right.”

At the West End Temple, after a Shabbat service that ended by honoring members of the congregation who were about to go to Israel, Stern offered a more tempered view. There were times when anti-Zionism “clearly is antisemitic,” he said, and other times when it was not. He noted that many Jews did not identify with Zionism or even see it as compatible with Jewish values. “Imagine if you were a Palestinian,” he said. “How would you think about Zionism?” Stern peered around the sanctuary and asked if anyone had ever spoken with a Palestinian about the subject. About half the hands in the room went up. “I don’t see Palestinians as antisemites at all when they say, ‘Look what Zionism did to me—it stopped my ability to control my land,’ ” he said.

For those wishing to hear an unswerving defense of Israel, Stern’s speech was likely disappointing. (As he left the stage, Margie leaned over to me and whispered, “They’ll never invite him back.”) But Stern has rarely agreed with Israel’s blind cheerleaders, even as he identifies as a Zionist. In the seventies, while attending Willamette University’s law school, in Oregon, he joined the National Lawyers Guild, where a debate arose about a resolution to recognize the Palestine Liberation Organization as the sole representative of the Palestinian people. This was not an idea that many supporters of Israel—which did not recognize the P.L.O. until 1993, when the Oslo Accords were signed—would have backed at the time. Stern voted for the resolution, a decision consistent with his leftist politics, which were reflected in the causes taken up by the law practice he launched in Portland. Among his clients was Dennis Banks, a founder of the American Indian Movement (AIM) who, in 1976, was arrested for the possession of illegal weapons. Stern would eventually tell Banks’s story in “Loud Hawk,” a gripping book about the case and a damning indictment of the bigotry that Banks and other members of the AIM movement endured.

But, if Stern has never been a dogmatic supporter of Israel, he has no more felt at home among its unsparing critics on the left. In 1982, after the outbreak of the Lebanon War, he resigned from the National Lawyers Guild. Stern opposed the war and even attended some demonstrations against it, but he bristled at the way some of his fellow-progressives spoke about Israel, which one colleague told him was behaving exactly like the Nazis. In its preoccupation with Jewish power, the discourse of some left-wing activists reminded Stern of the pamphlets he’d seen distributed outside the county courthouse in Portland by the Posse Comitatus, a right-wing extremist group that believed Jews controlled the media and banking system.

By 1989, Stern had begun working for the American Jewish Committee, where he would spend a quarter of a century, publishing numerous studies on antisemitism, including a book about Holocaust deniers. Stern speaks fondly of this period and of the person who hired him, Gary Rubin, who would go on to become the head of Americans for Peace Now, a left-wing nonprofit. Slowly, though, he grew alienated again, this time by the pro-Israel orthodoxy that he sensed taking hold at the A.J.C. In “The Conflict over the Conflict,” he describes learning, in 2005, that the organization was planning to issue a press release calling for Rashid Khalidi, a historian at Columbia University, to be removed from a training program for New York City public-school teachers, alleging that his presence would turn them into “Israel-haters.” When Stern reached out to some professors at Columbia, they told him that Khalidi, who is Palestinian, was a gracious colleague who invited pro-Israel faculty to address his class. He also learned that the A.J.C. was going after Khalidi because he had called Israel’s occupation of the West Bank “an apartheid system in creation”—a view that numerous Israeli politicians have echoed in recent years. (The A.J.C. declined to comment.) Stern was asked to draft the press release himself. He resisted doing so, which did not stop Khalidi from getting removed from the program, an act the A.J.C. praised. Stern later apologized to Khalidi for the incident.

A few years later, in 2011, Stern co-authored a letter that criticized outside groups for using the I.H.R.A definition to “silence anti-Israel discourse and speakers.” The letter was approved by the A.J.C.’s senior staff, but after publication it sparked a backlash from various activists and donors on the right. Their efforts paid off, prompting David Harris, the A.J.C.’s executive director, to withdraw support from the letter, calling it “ill-advised.”

Stern tells this story in “The Conflict Over the Conflict,” a work that is unlikely to please partisans. The book makes the case for bridging differences and recognizing nuance. It also describes Israeli-Palestinian history as an “ideal subject” to teach at universities, precisely because it is so divisive. At the West End Temple, Stern reiterated this belief. “On college campuses, students have an absolute right to expect they’re not going to be harassed, they’re not going to be bullied,” he said. “But to be disturbed by ideas is O.K.: we want students to be disturbed by ideas and to figure out how to think about them.”

Stern knows this view works against the prevailing culture on many campuses today. He’s also aware that advocates on both sides of the Israeli-Palestinian conflict can sometimes prefer to hear only opinions that mirror their own. In 2021, he experienced this firsthand, when the Zionist Organization of America and the Philadelphia chapter of the A.J.C. pressured Temple University’s Feinstein Center for American Jewish History to withdraw its support for a panel discussion about the I.H.R.A. definition which featured Stern and Joyce Ajlouny, a Palestinian American and the general secretary of the American Friends Services Committee, a social-justice nonprofit. Some time later, Stern’s participation in a Barnard event sparked yet more controversy, this time when Jewish Voice for Peace sought to deplatform him because he’d written the I.H.R.A. definition. “I’ve given the Z.O.A. and J.V.P. at least one thing to agree on,” he joked.

For Stern, the impulse to silence people with opposing views reflects a craving for “bright lines”—moral partitions that sort the good from the bad, making it easy to dehumanize the other side. He’s explored this theme in numerous books, including a 1995 study of the American militia movement that appeared nine days before Timothy McVeigh, an anti-government extremist, blew up a building in Oklahoma City, killing a hundred and sixty-eight people. The seductiveness of binary thinking, Stern says, is what attracts individuals like McVeigh to hate groups, where this proclivity takes extreme form. Yet its appeal is hardly limited to them. All human beings are susceptible to this urge, not least the members of his own tribe. “We crave simplicity,” he told the congregation at the West End Temple, “and we tend to downplay what our side is doing that’s bad.”