Psychologist John Monahan is a pioneer in the field of prediction who has influenced my work and life a great deal. In his beautifully written book, Predicting Violent Behavior, he begins by asking the simplest question: In which direction would this book fall if you let it go? The reader could technically state only that every other solid object he or she has let go of in the past has (eventually) fallen down rather than risen up or remained suspended. What allows for the prediction that this object, if released in the future, will also fall down is that we possess a theory—gravity—that can
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