Introduction: What Are You to Believe?

Before obtaining certainty we must often be satisfied with a more or less plausible guess.

—George Polya1

Try this sometime. Ask a friend, “Why do you believe what you believe? What sort of evidence persuades you that someone is right or that a product is good?” This question seldom elicits a careful, thoughtful response. Rather, it elicits silence and narrowed eyes. Most people think that their beliefs are shaped by logic and reason. Your friend will likely detect a whiff of insult in the question.

But our beliefs are fueled by much more than reason and fact. Yes, we are persuaded by solid evidence assembled into arguments that conform to principles of logic. But that’s true only for the messages that we ...

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