Note 7 — Chapter I · THE LOWER FORMS OF ABSTRACTION.

Herbert Spencer, Principles of Psychology. Vol. I., Part 2, Chapter II.—Bain (in the last chapter of Emotions and Will) says that nothing more fundamental can possibly be assigned as a mark of intelligence than the feeling of difference between consecutive or co-existing impressions. “There are cases, however, where agreement imparts the shock requisite for rousing the intellectual wave; but it is agreement so qualified as to be really a mode of difference.” For a review and ample discussion of this problem see Ladd’s Psychology, Descriptive and Explanatory, Chapter XIV. The earlier psychologists, in considering the “faculty of comparison” which acts by resemblance and difference, as primordial, had observed the same fact, although they described it in different terms.