For many years I taught a course titled “Current Controversial Issues and the Constitution.” Students often had radically different interpretations of the Constitution, and so, because I thought open debate was the best way to resolve disagreements, I strongly encouraged debate in the classroom.

But I had one hard-and-fast rule, “Whatever your position, however passionately you feel, you may not get mad.” Whenever I saw a student’s face get red or his or her voice get louder, I jumped in to keep the debate on an even keel. The rule worked. Disagreements were civil.

This got me to thinking: Why do bright people have such strong disagreements? Wouldn’t bright people generally agree? The answer is: often no.

Two intelligent individuals can discuss, say, 10 topics, agreeing on nine of them. But when it comes to the 10th, if it’s political, they diverge passionately. That is because rationality ends and passions kick in. They look at, say, 20 facts, then argue in retrospect—that is, the liberal focuses on 10 liberal views to the exclusion of conservative views, and the conservative focuses on 10 conservative views to the exclusion of liberal views.