 The first assumption, that \(p(\neg R \mid \neg B)\) is very high, seems pretty sensible. With all the non-ravens in the world, the probability that a given object will be a non-raven is quite high, especially if it’s not black. The second assumption is that \(p(\neg B\mid H)=p(\neg B)\). In other words, assuming that all ravens are black doesn’t change the probability that a given object will not be black. This assumption is more controversial (Vranas 2004). If all the ravens are black, then some of the things that might have been black aren’t, namely the ravens. In that case shouldn’t \(p(\neg B\mid H) &lt; p(\neg B)\) instead? On the other hand, maybe all the ravens being black doesn’t reduce the number of black things in the universe. Maybe it just means that other kinds of things are black slightly more often. Luckily, it turns out we can replace (ii) with less dubious assumptions (Fitelson 2006; Fitelson and Hawthorne 2010; Rinard 2014). But we can’t do with no assumptions at all, which brings us to two crucial points about confirmation and probability.