THE EXPERIMENTAL PROBLEMS set for the subjects in Chaps. 1 to 5 were designed to show a gradation in the sorts of difficulties overcome by the combinatorial method inherent in formal thinking and adolescent propositional logic. To conclude the first section, we should like to examine briefly another rather simple problem, one which has already been used in one of our previous studies; 2 it will serve to show how the stage III subjects utilize disjunctions and exclusions in integration with the entire set of binary operations. The problem is to determine why a metal bar attached to a nonmetallic rotating disk stops with the metal bar pointing to one pair of boxes instead of any other boxes placed around the disk; actually, the crucial pair contain several magnets concealed in wax. (Everything is placed on a board which is divided into sectors of different colors and equal surfaces.)

§ Stage I. Preoperational Disjunctions and Exclusions We need not refer to the responses of the youngest subjects (substage I-A), for they have been described in our previous study.