 In MWO, he says the a priori is knowable by the reflective and critical formulation of our own principles of classification, at least with respect to meaning connections explicitly before the mind (Lewis 1929, 287–8) and in AKV that a priori truths are certifiable by reference to meanings alone (and their relations like inclusion), and tested simply by what we can think or imagine being so (Lewis 1946, 35, 151–3). However, the only explanation he gives of why this should warrant us in thinking anything necessary or possible is that “as what we intend at the moment at least, a meaning seems to be as open to direct examination as anything we are likely to discover” (Lewis 1946, 145). It is this that warrants us as dismissing any apparent Euclidean triangle the sum of whose angles isn’t 180 degrees as either a mismeasurement or not a Euclidean triangle rather than a counterexample to Euclidean geometry. Barring certainty, there seems to be a half-acknowledged, non-inductive, basic principle of a priori credibility assumed here, to the effect that if on reflection on our concepts and meanings and classificatory intentions, we think A includes B, then we are at least prima facie warranted in thinking A does include B. In the next section, we shall see that Lewis isn’t in principle averse to non-inductively supported principles of prima facie warrant. 