Think of probability as a conserved quantity: there’s only so much to go around. As the number of details in a story goes up, the number of possible stories increases exponentially, but the sum over their probabilities can never be greater than 1. For every story “X and Y,” there is a story “X and ¬Y.” When you just tell the story “X,” you get to sum over the possibilities Y and ¬Y. If you add ten details to X, each of which could potentially be true or false, then that story must compete with 210 - 1 other equally detailed stories for precious probability. If on the other hand it suffices to
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