A careful reading of Cronbach and Snow's "Aptitudes and Instructional Methods" reveals that an aptitude-treatment-interaction (ATI) approach is not just a research methodology but is, in fact, a theory oriented philosophy of proposed revision in curriculum and instructional methodology. It is our contention that as a theoretical argument the ATI approach extends current practice in a desirable direction but that this approach, like those it is intended to replace, seriously limits the possibility of optimal adaptation to individual differences. This paper attempts to identify the theoretical premises of the Cronbach/Snow argument and to propose alternatives which are less limiting. A new theoretical methodology, based on these alternative premises, is described. It is suggested that the study of ATI's may be unnecessary for the optimal adaptation of instruction to individual differences. (Author)