 So the tracking account is a relevant alternatives approach. But why say that relevant alternatives accounts of knowledge are in tension with K? We will say this if, like Dretske, we accept the following crucial tenet: the negation of a proposition p is automatically a relevant alternative to p (no matter how bizarre or remote not-p might be) but often not a relevant alternative to things that imply p. For a relevant alternatives theorist, this tenet suggests that we can know something p only if we can rule out not-p but we can know things that entail p even if we cannot rule out not-p, which opens up the possibility that there are cases that violate K. For while our inability to rule out not-p stops us from knowing p it does not stop us from knowing things that entail p. And an example is ready to hand: the zebra case. Perhaps you cannot rule out mule; but that stops you from knowing not-mule without stopping you from knowing zeb. These points can be restated in terms of the conclusive reasons account. For Dretske, the negation of a proposition p is automatically a relevant alternative since condition CRA is automatically met; that is, it is vacuously true that: