by John Allen Paulos

About the author: John Allen Paulos, a professor of mathematics at Temple University in Philadelphia, Pennsylvania, and adjunct professor of journalism at Columbia University in New York, is author of Innumeracy, a book that explains how numbers can lead to misinformed government policies, confused personal decisions, and susceptibility to pseudoscience.

Let’s start with a basic question: What is the purpose of the battle against terrorism? One answer, perhaps reflecting how most Americans see things, is that we want to feel safe. As philosopher Thomas Hobbes knew, what people want most from the state is protection, not freedom. To this end, and since terrorists appear relatively invulnerable to the usual deterrents, it follows that we would ideally intercept them before they carry out their attacks.

This is part...