 In MWO Lewis argues that probable empirical knowledge requires, on pain of infinite regress, some a priori knowledge of analytic principles explicating our concepts, the logical relationships among them, and the criteria for applying them to experience and determining what is real and what isn’t real. Moreover, this knowledge must be “more than probable” and “certain”, which suggests that they have a degree of warrant greater than the degree of probability empirical considerations could yield (Lewis 1929, 311–12, 317, 321). In AKV, he also says that mathematical and logical cognitions “may be certain” (Lewis, 1946, 29), at least in certain cases, but he worries how they can refer to anything beyond the cognition itself, and be classified as knowledge, if their truth is simply the unthinkability of their falsity, given our concepts or way of thinking. The answer is that we can be mistaken about what is implicit in our concepts. We can fail to observe what is implicit in our concepts and what their adoption consistently commits us to in our thinking, and that contrast with error allows him to classify a priori apprehension as knowledge. Nonetheless, any of these mistakes is “intrinsically possible of correction merely by taking thought of the matter” (Lewis 1946, 155) without empirical investigation. The correctness of the principles governing our concepts can be known a priori, independently of confirmation in experience, in so far as they can be certified or assured simply by analysis of meaning or reflection on the content of our concepts and our explicative principles (Lewis 1946, 151,165). So, quite apart from issues about certainty, the degree to which we are warranted in our a priori apprehensions needn’t correspond to and isn’t a function of their probability on the total set of empirical considerations (cf. Lewis 1926, reprinted in Lewis 1970, 243–4). 