The General Conditions of Knowledge: External Conclusiveness
If all justification of confidence had to be infallible then conditions (1), (2), and (3) of our general definition would be sufficient for knowing that something is the case. But the possibility of fallible justification for a claim to know brings with it the possibility that conditions (1)–(3) should be satisfied and yet S fail to know that p, because the facts giving him his fallible justification do so only because they are protected by his ignorance of, or false but justified beliefs about, certain other facts, ones such that were he to learn of them the question of whether or not p would be entirely reopened for him (if he were being reasonable), his justification for being confident that p would be wiped out. Condition (4) must rule out just all such possible circumstances in which (1) and (2) are satisfied and (3) is satisfied by a fallible justification for claiming to know that p and yet S does not in virtue of that justification know that p. Let us speak of a circumstance that falsifies condition (4) with respect to a particular justification for claiming to know that p as defeating that justification for claiming to know that p. Let us consider first kinds of circumstance that can defeat non-inferential justifications and then kinds that can defeat inferential ones.
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