How many butterflies have you never seen? It sounds like a riddle from Alice's Adventures in Wonderland, a patently silly question without any rational answer. Yet perfectly sensible people ask similar questions all the time. Military intelligence analysts want to know how many tanks the other side has, without much hope of being allowed to count them. Health planners need to know how big a drugs problem a city faces, but drug users aren't exactly keen to talk to pollsters. These are sensible questions, but can they ever have sensible answers? Surely no one can tell you how many butterflies—or tanks, or drug addicts—are out there that you have never seen?
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