his distinction between negative and positive doubt points to a much later era. The debates of the time of Laplace on the principle of insufficient reason never satisfactorily resolved whether the probability of a coin’s landing heads is a half because the coin is symmetrical, and hence there is no reason to prefer heads to tails, or because many throws of coins have been observed to produce about half heads and half tails. Keynes’ chapter on the weight of evidence shows that we are still no closer to explaining the difference between the probability of a hypothesis in the two cases in which
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