Ms. Jacoby said, something different is happening: anti-intellectualism (the attitude that “too much learning can be a dangerous thing”) and anti-rationalism (“the idea that there is no such things as evidence or fact, just opinion”) have fused in a particularly insidious way.

Walking home to her Upper East Side apartment, she said, overwhelmed and confused, she stopped at a bar. As she sipped her bloody mary, she quietly listened to two men, neatly dressed in suits. For a second she thought they were going to compare that day’s horrifying attack to the Japanese bombing in 1941 that blew America into World War II:

“This is just like Pearl Harbor,” one of the men said.

The other asked, “What is Pearl Harbor?”

“That was when the Vietnamese dropped bombs in a harbor, and it started the Vietnam War,” the first man replied.

At that moment, Ms. Jacoby said, “I decided to write this book.”