 The tracking accounts permit counterexamples to K. Dretske’s well-known illustration is the zebra case (1970): suppose you are at a zoo in ordinary circumstances standing in front of a cage marked ‘zebra’; the animal in the cage is a zebra, and you believe zeb, the animal in the cage is a zebra, because you have zebra-in-a-cage visual percepts. It occurs to you that zeb entails not-mule, it is not the case that the animal in the cage is a cleverly disguised mule rather than a zebra. You then believe not-mule by deducing it from zeb. What do you know? You know zeb, since, if zeb were false, you would not have zebra-in-a-cage visual percepts; instead, you would have empty-cage percepts, or aardvark-in-a-cage percepts, or the like. Do you know not-mule? If not-mule were false, you would still have zebra-in-a-cage visual percepts (and you would still believe zeb, and you would still believe not-mule by deducing it from zeb). So you do not know not-mule. But notice that we have: