 In MWO Lewis (1929, 254) also thought that the a priori extended to fundamental laws of nature defining basic concepts like mass or energy or simultaneity, and thus included some of what are typically regarded as the basic principles of a scientific theory. Further, besides criteria like convenience and conformity to human bent, pragmatic considerations mentioned in MWO (Lewis 1929, 267) include factors like intellectual simplicity, economy, comprehensiveness, and thus the overall achievement of intellectual order. However, unlike Wilfrid Sellars and many others in the latter half of the 20th century, Lewis never recognized such factors as criteria of empirical justification. The reason seems to be that Lewis (1936b, reprinted in Lewis 1970, 286) didn’t think that these factors make a hypothesis any more probable, in contrast, presumably, to conformity to standard criteria of induction: “What such simplicity and convenience determine is not truth or even probability but merely simplicity and convenience, which have their reasonable place in the choice of working hypotheses when no more decisive criterion is presently at hand”. At the same time, he thought that the acceptance and rejection of scientific theories wasn’t entirely empirical. The choice of a system of concepts and a priori explicative principles to apply to experience and to interpret it in the light of is determined by pragmatic considerations, not truth or probability. The simpler set of scientific concepts and explicative principles is no more true or likely to be true than the simpler logic is more true or valid. Moreover, pragmatic considerations might lead us in the face of experience to abandon our scientific concepts and a priori principles explicating them without making the latter empirically unwarranted or any less a priori warranted.