 The first point is that Nicod’s Criterion fails. Assumptions like (i) and (ii) of the Raven Theorem don’t always hold. In fact, in some situations, discovering a black raven would actually lower the probability that all ravens are black. How could this be? The trick is to imagine a situation where the very discovery of a raven is bad news for the hypothesis that all ravens are black. This would happen if the only way for all the ravens to be black is for there to be very few of them. Then stumbling across a raven would suggest that ravens are actually plentiful, in which case they aren’t all black. Good (1967) offers the following, concrete illustration. Suppose there are only two possibilities: