Good Explanation, Poor Prediction

Let's say you're overweight, have a bad diet, and never exercise. Let's say you go to the doctor for an annual checkup, and - for years - he tells you, "You've got to lose some weight, change your diet, and increase your activity level, or else you're going to acquire type 2 diabetes."

But, for years, you choose not to change your lifestyle and, for years, you fail to acquire type 2 diabetes. From time to time, some of your friends say, "Doctors like yours don't know anything about diabetes. They've been predicting that you'd become a type 2 diabetic, and yet it still hasn't happened yet!"

Then, one day, it happens. You're devastated. In a moment of weakness, your doctor says, "For years, I've been warning you that your poor health habits would result in type 2 diabetes - you should have listened to me."

Suppose you were to respond to your doctor as follows:

"True, for years you warned me and for years it never happened. So how useful was your warning? Not very. Clearly I have now acquired type 2 diabetes, but that doesn't mean your theory about weight, diet, and exercise is correct. What, if anything, would cause you to second-guess your beliefs?"

The moral of this story is: Sometimes ideas - even ideas we think are completely uncontroversial - have strong explanatory power but poor predictive power.

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