There are as many answers to this question as there are terrorist groups, and everyone from clerics to caustic cab drivers seems to have a confident opinion on the subject, as though the interior world of terrorists can be easily mined and mapped. But this confidence is often misplaced, given how little scholars actually know about terrorism and the people who are involved in it. It also betrays an epic obliviousness about just how difficult it is to access the internal, subjective desires and emotions that shape the outer world. Instead of asking why people join terrorist groups and commit terrorist atrocities, a more worthwhile starting point for explanation is to ask how.

One culturally prevalent answer to the why question is that terrorists are “driven” or “pushed” to do it, and that the decisive driving or pushing agent is pathology. This answer has evolved in recent years in line with advances in knowledge and moral sensibilities. In terrorism studies in the late 1960s, it was not uncommon for scholars to conceive of pathology as a psychological abnormality or affliction rooted inside the individual. Since the 1980s, this idea has fallen into disrepute, and the scholarly consensus now holds that the roots of terrorism lie not in the individual, but in the wider circumstances in which terrorists live and act.

The scholarly consensus on violence has a lot going for it. It humanizes the perpetrators of violence by insisting on their ordinariness and contextualizing their actions. It obliges people to reflect on their own possible shortcomings and vulnerabilities, and how, in different circumstances, they too could do monstrous deeds. And it compels people to recognize that they do not act in a social vacuum, and that what they think, feel, and do is powerfully shaped by the broader historical circumstances in which they are compelled to live and act. Moreover, Westernized jihadists, as a recent report cogently suggested, assuredly are alienated and feel that they do not belong in a secular world that often mocks and challenges their religion and identity as Muslims.

But the consensus can’t divest itself of the idea of pathology. Rather, it simply relocates the notion, tracing the causes of violence to pathological “background factors” operating on the violent. No doubt this is a more illuminating and edifying narrative than that sketched out in earlier psychological accounts. But its explanatory power is limited, because, as the eminent sociologist Jack Katz has convincingly argued, “whatever the validity of the hereditary, psychological, and social-ecological conditions of crime, many of those in the supposedly causal categories do not commit the crime at issue, … many who do commit the crime do not fit the causal categories, and … many who do fit the background categories and later commit the predicted crime go for long stretches without committing the crimes to which theory directs them.” Or as the British writer David Aaronovitch once joked, “Why don’t black lesbians blow up buses? Aren’t they alienated enough?”

One of the most sensitive and profound explorations of terrorism in recent years comes not from a scholar, but from a novelist. Philip Roth’s American Pastoral is a murder mystery in which the focal point is not the who, but the why. The protagonist, Seymour Levov, is a successful businessman whose 16-year-old daughter Meredith (“Merry”) blows up a post office to protest the Vietnam War, killing a bystander. All Seymour can think about is why Merry did it. She was an adored only child who grew up in a privileged and decent family in the idyllic hamlet of Old Rimrock, New Jersey. Seymour is desperate to locate “the wound” that caused Merry’s violence. Was it her stutter? Was it that anomalous kiss on the mouth he gave her one summer when she was 11 and he 36? Or was it the mysterious firebrand Rita Cohen who radicalized her?

As the novel progresses, Seymour’s disbelief gives way to clarity. But it is a negative clarity. “He had learned the worst lesson that life can teach—that it makes no sense,” Roth writes. He had learned that his daughter “was unknowable,” and that “there are no reasons,” that “reasons are in books.” In capturing Seymour’s efforts to understand the disaster that befalls his family, Roth holds up to scrutiny conventional efforts to explain terrorism—and exposes just how imaginatively cramped and simplifying they can be.

Terrorism scholar John Horgan has made a similar point. “The most valuable interviews I’ve conducted [with former terrorists] have been ones in which the interviewees conceded, ‘To be honest, I don’t really know,’” he writes. “Motivation is a very complicated issue. To explain why any of us does anything is a challenge.” It’s a challenge further compounded by the fact that some actions are informed by multiple motives, and even if these can be reliably identified it is often difficult to disentangle them and calculate their respective causal weight.

As Horgan suggests, a more manageable and useful question to ask about terrorism is not why, but how—and when and where? How did this specific person come to join this specific organization? What networks helped facilitate the act of joining, and where and how were these networks accessed or sought out?

This isn’t to suggest that the why question should be abandoned, but rather that those who ask it better appreciate the magnitude of the question and acknowledge the possibility that some momentous life decisions will remain forever opaque and mysterious—not only to outside observers, but also to the people who take them and must live with the consequences.