Gettier's examples show that someone can believe a truth, have good reasons for believing it, and yet lack knowledge.1 How can this be? One explanation is that a man's good reasons can depend upon or include false beliefs. Another possibility is that a justified true belief can fail to stand in an appropriate causal relation to the fact which makes the belief true. Both explanations fit Gettier's examples, but perhaps neither is sufficiently general: each assumes, as indeed Gettier's exposition does, that someone is adequately justified or not in virtue of evidence he actually has for a truth. Might not a man's good reasons be said to be defeated, undermined, or rendered inadequate, by further evidence he would, but happens not to, take into account? This is the possibility adverted to by certain defeasibilist analyses of knowledge and nonknowledge. Such analyses grant that Gettier's subjects are justified in believing a truth, but they explain in a distinctively general way the fact that those subjects lack knowledge. The lack ensues from incomplete justification. If someone's actual justification for a true belief admits of supplementation or correction that would disclose evidence against the belief, then one's actual basis for believing is incomplete. Conversely, if correction or supplementation were not to detract from one's reasons, one is actually completely justified, and one's true belief is knowledge-ranking. In Gettier's cases, conditions of complete justification fail to be met. But complete justification might be absent, too, where one's reasons involved no falsehoods, and where one's true belief was non-deviantly caused. In those circumstances it could still turn out that exposure to further evidence would warrant the retraction of some actually held true belief. Defeasibilism thus seems to alert us to, and to account for, cases of non-knowledge that Gettier did not prepare us for.