To be convicted of a crime in the United States, a person must be proven guilty "beyond a reasonable doubt." But what is reasonable doubt? Even sophisticated legal experts find this fundamental doctrine difficult to explain. In this accessible book, James Q. Whitman digs deep into the history of the law and discovers that we have lost sight of the original purpose of "reasonable doubt." It was not originally a legal rule at all, he shows, but a theological one.

The rule as we understand it today is intended to protect the accused. But Whitman traces its history back through...