 Despite their lack of theoretical certainty, the beliefs we form by applying concepts to experience may count as knowledge so long as they are true and sufficiently warranted or justified. The members of a set of beliefs that already have some degree of confirmation or antecedent probability in relation to present and past experience may become even more credible if the antecedent probability of any one would be increased by assuming the others as given (Lewis 1946, 187, 338, 349, 351, 352). The congruence of a mature system of beliefs which exhibits this complex interlocking pattern of probability relations to each other and to experience helps to explain how many of these beliefs can rise to the standards for knowledge and be practically certain enough to be counted on in action. 