Warning: This column may contain material you disagree with or find offensive. It may provoke a strong reaction, making you feel angry or exposed. Of course, you can log off or turn the page. But this is the opinion section of a general-interest newspaper. Shouldn't you expect to find provocative, even threatening ideas? And shouldn't other readers be able to see this column without a cautionary note that it might do them harm?

Something similar is happening on college campuses, where reasonable concern for students who may have suffered terrible traumas has morphed into a serious threat to intellectual freedom. Increasingly, students are expecting "trigger warnings" to be issued before they are asked to read certain texts or view course material that may be troubling. It can be something as raw as a graphic rape scene or a bloody wartime battle, or more conceptual, such as themes of racism or oppression. At some schools, students want to be allowed to skip a class or reading if they fear it will trigger a stressful reaction.

The criteria for the warnings are varied and ill-defined. At Rutgers University, Virginia Woolf's novel "Mrs. Dalloway" was targeted for a warning because it contains thoughts of suicide. At Oberlin College, students requested one for Chinua Achebe's "Things Fall Apart" — a hardy perennial on freshman reading lists — because of its treatment of colonialism. Trigger warnings have been proposed for "The Great Gatsby" and "The Merchant of Venice" because they depict violence, misogyny, or racial slurs.

Trauma triggers are very real for some students — rape victims and war veterans, for example. Any concerns they bring to a professor ought to be treated with respect. But to slap a blanket warning on a piece of literature is a short step from an effective ban. Who can doubt that professors will self-censor their course material rather than risk collecting too many "triggers?" What college wants to appear insensitive to vulnerable students?

Many academics see the warnings as an attack on free expression, and for good reason: The alerts by their nature single out some content as better or worse than others. They pre-judge course material as dangerous or objectionable, skewing the students' experience in advance. As with health notices on cigarette packs, the warnings signal that the material is best avoided.

Trigger warnings aren't new; they are common on the Internet, where they alert readers to a range of potentially upsetting material from common profanity and insensitive jokes to depictions of drug abuse, eating disorders, even spiders. But they are especially worrisome on college campuses, where exposure to a free exchange of ideas is paramount. "When a student opts for a liberal arts education, they have opted to jump into the cauldron of life," said attorney Harvey Silverglate, a fierce advocate for freedom of thought on campus. "You should expect to be occasionally very disturbed. That is actually part of the education."

Much of the focus on content warnings grows out of a concern for marginalized groups, whether minorities, the disabled, or anyone not in the "dominant culture." Feminist studies in particular have promoted them as a way to make women feel safer in a sometimes hostile campus environment, which can and does include sexual assault. But there are as many potential triggers as there are students. It's a practical impossibility to protect against all of them.

Nor should we try. Nancy Gertner, a former federal judge and professor at Harvard Law School, says students have asked her to disclose whether an exam in her criminal law course would contain any triggers for rape victims. She has refused. "I have a feminist objection to the notion that women need to be inoculated against certain issues," she said. "Women need to engage, to come to grips with these issues." The university should prepare students for the rest of life. "There are no more trigger warnings the minute they graduate," she said.

Abetted by the Internet, Americans have been drawn deeper into like-minded communities of interest, where being confronted by challenging or contrary ideas is increasingly rare. Of course these subjects are painful. But exposure to society's horrors is what wakes up our compassion and humanity. Students — all of us — need to learn about evil before we can ever hope to defeat it.