“I became a historian in part because I really believe in ambivalence,” says David Nirenberg, the Deborah R. and Edgar D. Jannotta Distinguished Service Professor in History and interim dean of the Divinity School.

Beginning with his dissertation at Princeton University—the basis for his first book, Communities of Violence: Persecution of Minorities in the Middle Ages (Princeton University Press, 1996)—he has argued that religious violence in the Middle Ages had both “associative and dissociative” effects, enabling the existence of a pluralistic society while also containing the seeds of its destruction. For that project, he drew on the extensive archives of the Crown of Aragon, a kingdom based in what is now eastern Spain from the 12th to the 18th centuries, in which Jews, Christians, and Muslims coexisted in high numbers.

A key example Nirenberg analyzed at length is the violence that occurred for centuries in Iberia and other parts of Europe during Holy Week, the week immediately preceding Easter in Christianity. Alongside the ritual reenactment of the Passion (the capture and crucifixion of Jesus), Holy Week in many places just as reliably featured the harassment of the local Jewish community, predicated on their supposed collective responsibility for Jesus’s death. Although Jews in Aragon were officially protected by the king, each year Christian clerics would incite their flocks to steal Jewish artifacts, stone Jewish dwellings, and sometimes wound or even kill Jews. Muslims, as fellow believers in Jesus’s revelation—Jesus is a prophet, but not a divinity, in Islam—would sometimes join Christians in these attacks.

To Nirenberg, the most remarkable fact about Holy Week violence is how constrained it usually was. Overt aggression was limited. “Stoning” was usually of buildings, or of the walls of the Jewish quarter, not people. Much of the harassment, spearheaded by children, was more prankish than bloodthirsty in tone. Nirenberg’s conclusion: this constrained aggression represented an annual reaffirmation of Christians’ historical ties to, as well as differences from, the Jewish people, channeling animus into ritual so as to allow for the communities’ peaceful coexistence for most of the year. But on the rare occasions when that violence jumped its channels, it could be disastrous, resulting in mass murder and mass conversion.

Nirenberg was not condoning violence. Nor was he denying that even the most constrained forms of Holy Week violence were terrifying to those who suffered it. His point, rather, was that there was something ambivalent in it, a way of affirming “the continued existence of Jews in a Christian society, while at the same time articulating the possibility of and conditions for their destruction.”

By the time a new edition of Communities of Violence was released in 2015, Nirenberg had gone on to publish Anti-Judaism: The Western Tradition (W. W. Norton, 2013), a sweeping “account of the labor done by Judaism in the workshops of Western thought”—notably, the labor of serving as a target for projections over millennia, a figure of thought used by Christians, Muslims, and many other cultures as they articulated their fears and their ideals.

He had also returned to Iberian archives to write Neighboring Faiths: Christianity, Islam, and Judaism in the Middle Ages and Today (University of Chicago Press, 2014), further articulating the idea of religious communities bound by “a fundamentally ambivalent form of neighborliness,” always defining themselves in relation to one another, in intimate, tolerant, and violent ways that cannot easily be parsed.

With these and other works examining the lives of faith communities, Nirenberg has emerged as a leading scholar of the Abrahamic religions.

As a Muslim, Christian, or Jew seeking guidance from prophets and scriptures of the distant past, figuring out what you’re supposed to do, how you’re supposed to live, is a kind of historical work,” Nirenberg says. Claims about what a passage of a sacred text might mean, or what we should learn from the teachings of a founding figure who lived long ago, are often made by believers in historical terms.

The history of religions is also important to nonbelievers. A case in point is religious hate speech. Last year Nirenberg consulted with Facebook on its hate speech policy, a troubling issue for the tech giant given the sheer volume of invective it plays host to, including a high-profile incident in 2017 when the platform was instrumental in fanning the flames of anti-Muslim persecution by Buddhist nationalists in Myanmar. The United Nations issued a 2018 report on the country that called Facebook “a useful instrument for those seeking to spread hate.”

Nirenberg was asked to bring a historical perspective. Whether it is anti-Semitic attacks in the United States or the 2019 mosque attack in Christchurch, New Zealand, it’s clear that religious ideas from the past are being deployed on social media to mobilize violence, something Facebook’s policies forbid. But when it comes to deciding what expressions should be blocked as religious hate speech, he says, “it’s not always easy to figure out where to draw the line between material that advocates violence and material that expresses a religious tradition or ideal.”

He gives the example of the 2018 Tree of Life synagogue shooter in Pittsburgh, whose social media account profile included a quote from the Gospel of John—John 8:44. In that verse, Nirenberg says, Jesus refers to Jews and Pharisees as children of Satan. “The passage has been used to incite anti-Jewish violence for centuries. Does that mean John 8:44 and other anti-Jewish passages of the New Testament should always be flagged on social media as hate speech? That would raise serious questions about religious freedom.” The same, he adds, could be said of the sacred texts of many other faiths. History “gives us a certain purchase on the present,” Nirenberg says, but “it doesn’t give us rules for how to separate our hatreds from our faiths, values, and ideals.”

What is clear is that the shooting came as part of a renewed wave of anti-Semitism in recent years, which may have gained further traction in the past few months. Facing a pandemic and global economic meltdown, people are indulging in conspiracy theories to make sense of a chaotic world, and as Nirenberg’s scholarship details, the Jews have long been cast in the role of a nefarious, behind-the-scenes orchestrator of global events. At the same time, nationalist movements in the United States and Europe are trafficking in the timeworn idea of Jewish people as cosmopolitan outsiders, “globalists” alien to the true essence of national cultures.

Nirenberg told the New York Times in 2019 that the “electoral utility of anti-Semitism feels new to me, newly flexible, and therefore newly dangerous.” He cautioned in a New Yorker interview in January that these attitudes can be found anywhere: the right accuses the left of anti-Jewish sentiment for its criticism of Israel, while the left points to White nationalism on the right, but “the real danger is imagining that it is only the other [side] where anti-Judaism is doing its work.”

“We historians hate the word ‘unprecedented,’ because it threatens to put us out of work,” Nirenberg said, kicking off a lecture about plagues and faith—part of a series of virtual Harper Lectures offered this past summer as COVID-19 traversed the globe. “If something is unprecedented, then the study of the past has nothing to offer the present.”

Among the precedents to COVID-19 Nirenberg discussed were multiple outbreaks of plague, including one that eradicated about half of Europe’s population in the 14th century; the scourge of smallpox brought by colonists to the Americas, which may have killed 90 percent of the Indigenous inhabitants of the Western Hemisphere; and the influenza pandemic of 1918 that resulted in tens of millions of deaths worldwide.

UChicago’s own Core curriculum, he pointed out, is marked by pestilence: “the plague that opens the Iliad, or the plague that struck the Israelites in Exodus, both of which, by the way, precipitated debate and protest against authorities—against Agamemnon and Moses, respectively.”

Today as ever, the turmoil of an outbreak is compounded by our lack of understanding. Science may have improved, but a new pandemic can still trigger what Nirenberg calls “ground-zero empiricism,” borrowing a term from historian of science Lorraine Daston of the John U. Nef Committee on Social Thought. Confronted with a new disease, we are thrown back on direct observation, piecing together patterns, our public health efforts proceeding in fits and starts.

Another thing that hasn’t changed is that we look to religion to fill the gaps, or at least to provide a framework for making sense of human suffering. Nirenberg cites a nationwide poll he and his colleagues conducted last spring in collaboration with the Associated Press-NORC Center for Public Affairs Research. Over one quarter of the respondents said their faith has been strengthened by the pandemic, and 63 percent said that God is using the coronavirus to send a message to humanity about how to live. “This pandemic theodicy is fascinating,” says Nirenberg. “I studied the medieval religious movements occasioned by the Black Death in my dissertation. I never imagined that 30 years later I could do surveys about similar phenomena in my own society.”

Still, the same poll showed strong support for restrictions on in-person church services to curb infection rates, despite the fact that most respondents also considered religious freedom an important political right. Heartened by this mix of responses, Nirenberg again notes a kind of ambivalence—not in the negative sense of vacillating or lacking conviction, but in the positive sense of maintaining a balance or equilibrium among opposing forces.

There are tensions between ideals in every culture, he says, which is why we need to cultivate an important political virtue: that of resisting the tendency “to amplify tensions between different rights by turning them into zero-sum conflicts.”

In his short story “Blue Tigers,” Jorge Luis Borges describes a baffling find: a stash of circular blue pebbles, located by an intrepid Scottish professor outside a remote village in colonial India, that cannot be counted. Count them twice and the number differs. Spill them on the ground and they seem to multiply or diminish. Mark a pebble to track it and the markings disappear.

The problem, in mathematical terms, is that the blue pebbles violate the principle of identity: they are not identical to themselves. Our protagonist, an expert on logic, finds this result intolerable—the pebbles become a nightmarish obsession.

Nirenberg is less put off by the rascally rocks. In fact, with the exception of certain mathematical objects, he denies that anything is a stable, discrete, countable entity. Or rather, “everything is from one point of view or another a blue pebble or a countable pebble, and it’s up to us what approach we want to take, depending on the question we want to ask.” Even an actual pebble—a normal, non-Borgesian one—can be viewed as an open-ended process without clear boundaries. Stick it on an abacus, and the pebble is eminently countable. Reduce it to chemical analysis, or zoom out to a cosmological timescale, and the pebble as such disappears. How we think about it depends on our purposes, contexts, and forms of attention.

Bearing this point in mind, Nirenberg has recently coauthored a book, forthcoming from the University of Chicago Press, that defends the freedom to approach objects of inquiry—whether physical objects, social phenomena, or our own minds—as countable or “blue” pebbles. It’s a philosophical point, but it’s also a point about academic and human freedom, that is, our “basic freedom to decide under what kinds of conditions we want to treat our objects of knowledge.”

While the book explores the power of mathematics and quantification, it aims to understand that power in relation to many other forms of thought and knowing, among them dreams, religion, music, and poetry. Nirenberg sees the book as a call to attend to threatened aspects of the human, not as a defense of the humanities. But it is also an argument for the ongoing importance of humanistic fields like history, literature, and philosophy.

Nirenberg describes the perspective of the book, titled Uncountable: A Philosophical History of Number and Humanity from Antiquity to the Present, as one of ambivalence: “There are plenty of books out there on the evils of number, and there are plenty of books out there on, you know, the limitations of the humanities,” he says. “What I’m trying to do is hold these things together.”

He sees the book as similar in spirit to his work on religion. Both are attempts to grapple with the legacy of human beings’ most fundamental concepts: “How we think about number and its powers affects everything humans have done.”

The book’s other author, Ricardo Nirenberg, is not a historian but a retired mathematics professor—and David Nirenberg’s father. The book is an intergenerational as well as interdisciplinary affair.

Father and son’s formal collaboration began in 2011 when they cowrote an article on the philosophical implications of set theory for Critical Inquiry, but they have been thinking together, across different forms of knowledge, for a long time. “He made me memorize the Odyssey, Book I, in ancient Greek; he taught me Euclidean geometry,” David recalls of his childhood.

The set of the Nirenbergs increased by two when David and his twin brother, Sergio, were born, soon after the family immigrated to the United States from Argentina. After bouncing around the globe for a few years—Ricardo held a series of academic appointments in Europe and the United States while taking the family back to Argentina each summer—they settled down in Albany, New York, when David was about seven.

David’s mother Isabel, a computer programmer, worked on experiments carried out on spacecraft in the late 1970s before making a career in information technology at the State University of New York at Albany, where Ricardo was a professor. In the 1980s, she consulted for Time Life Books to help produce some of the first how-to guides for home computers. Along with Ricardo, she has also run the online literary journal Offcourse since 1998.

Living in Upstate New York, David understood that his immigrant family was not the norm. His response was to embrace being different. “My twin brother took one route into this experience of becoming American … baseball and hockey and stuff like that,” he says. “And I took the other route, which was to try to remain exotic.” He became addicted to Argentine tea—maté, which he still drinks regularly—and increasingly attracted to the Spanish-speaking world.

This posture went beyond adolescent self-styling and helped fuel his budding scholarly interests. “Not sounding like everyone around me, having a different language at home—it created a kind of interest in multiculturalism.”

But biophysics, not history, was his first path when he arrived at Yale for college. “I became a medieval historian because I heard a fantastic lecture on the history of the number zero,” he says, grounding his scholarly origin in a chance event that, in retrospect, lends a full-circle quality to his new book with his father.

Nirenberg draws inspiration from a variety of sources—even administrative roles. A man of many titles, he is interim dean of the Divinity School and former dean of the Division of the Social Sciences. He was executive vice provost. He was founding director of the Neubauer Collegium for Culture and Society. He sits on boards. He doesn’t sit still.

He says his proudest administrative achievement was serving as founding faculty director of the Neubauer Collegium from 2012 to 2014. It was “a marvelous experience, working to create a space in which people could come together in pursuit of ideas.”

A benefit of serving as dean of the social sciences was how it encouraged him to read his colleagues’ research in a systematic fashion across all the division’s disciplines. Every faculty member submits an annual report with samples of their scholarship—and he would read them all. “My administrative role proved invaluable in writing the chapter on the social sciences in this book with my father,” he says, citing guidance from economists James Heckman and Lars Peter Hansen about such topics as policy invariant structural parameters and transitive preferences, as well as discussions with psychologist Leslie Kay about olfactory functioning in rats.

Teaching has also been fruitful for his research. In Spring Quarter 2009, participating in the inaugural year of the University’s Study Abroad program in Jerusalem, Nirenberg had the opportunity to teach a group of students that was a microcosm of the faith communities he studies: a third Christian, a third Muslim, and a third Jewish. The students began deeply suspicious of each other, but he says they opened up in a remarkably short time. “Within a week they were wondering how they could ever have thought that they couldn’t talk to each other.”

Although he had spent much scholarly effort “to deflate the idea that if we all simply knew more about each other we would all get along,” the classroom discussions made him more hopeful about the power of fostering mutual understanding, especially through the study and teaching of history. The experience inspired him to write Neighboring Faiths in a more optimistic and constructive spirit than Communities of Violence.

Historians usually focus on the critical and deconstructive rather than on the constructive sides of their work, he says. But both sides are real, and “the stories and histories we tell can do a lot to either set us apart or bring us together.”