Suppose that there are several pieces of evidence, e1, e2 and e3 each of which would fit in with a hypothesis h, but each of which, on its own, is explained with less initial improbability on some other grounds, say by g1, g2, and g3 respectively. Yet if the improbability involved in postulating h is less than the sum of the improbabilities involved in the rival explanations g1, g2, and g3, though it is greater than each of these improbabilities separately, the balance of probabilities when we take e1, e2 and e3 together will favour the hypothesis h. It is important that it is just the one
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