 Robert C. Moore (Moore 1985) pointed out that we sometimes infer things about the world based on our not knowing certain things. So, for instance, I might infer that I do not have a sister, since, if I did, I would certainly know it, and I do not in fact know that I have a sister. Such an inference is, of course, defeasible, since if I subsequently learn that I have a sister after all, the basis for the original inference is nullified.