The flexibility of a rod depends on the material it is made of, its length, its thickness, and the form of its cross-sections. All other things being equal, the degree to which it bends varies as a function of the weight that is placed at its tip. To study the reasoning processes mediating the separation of variables and the verification of their respective roles, it seemed worth while to give our subjects a problem involving much greater empirical difficulty than the earlier ones, though not requiring for its solution concepts essentially more complex. In the case of floating bodies, we have just had a glimpse of the importance which the schema “all other things being equal” plays in hypothetico-deductive thinking. But the interference of five distinct variables, as in the flexibility problem, furnishes a situation particularly favorable for the study of the formation of this experimental schema and of the formal operations which it presupposes, for if a complete solution is to be attained each factor must be varied independently and the others held constant. 2