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gjm comments on Conservation of Expected Evidence - Less Wrong
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Can someone tell me if I understand this correctly : He is saying that we must be clear before hand what constitutes evidence for and what constitutes evidence against and what doesn't constitute evidence either way?
Because in his examples it seems that what is being changed is what counts as evidence. It seems that no matter what transpires (in the witch trials for example) it is counted as evidence for. This is not the same as changing the hypothesis to fit the facts. The hypothesis was always 'she's a witch'. Then the evidence is interpreted as supportive of the hypothesis no matter what.
You don't necessarily have to figure it out beforehand (though it's certainly harder to fool yourself if you do). But if X is evidence for Y then not-X has to be evidence for not-Y.
And yes, one thing that's going wrong in those witch trials is that both X and not-X are being treated as evidence for Y, which can't possibly be correct. (And the way in which it's going wrong is that the prosecutor correctly observes that Y could produce X or not-X, whichever of the two actually happened to turn up, and fails to distinguish between that and showing that Y is more likely to produce that outcome than not-Y, which is what would actually make the evidence go in the claimed direction.)
Did anyone say it is? I'm not seeing where.