Thus in every act of reproducing a past mental experience, there is a complication, involving both contiguity proper and similarity proper. When the similarity amounts to identity, as when a new impression of a thing puts us in the track of the old impressions of the same thing, the effect is so sure, so obvious, so easily arrived at, that we do not need to think of it, to make a question of it. It does not prevent us from regarding the operation of recalling a name when we see the thing, or recalling a thing when we hear the name, as pure contiguity. The strength of the coherence may be deficient, and the restoration may fail on this account; it can never fail on account of insufficient similarity. No inconvenience will arise from speaking of this case as if it were Contiguity and nothing else.
The situation of Similarity in Diversity is quite distinct. The diversity obstructs the operation of similarity; we cannot be sure that the new shall put us on the track of the old. It is always a question whether such similarities shall be felt at all; whether we shall experience the flash, the peculiar consciousness, of agreement in difference. It is a farther question, whether the internal coherence of the thing identified is enough to restore it in completeness. This last step may be allowed to be a case of proper contiguity; while the flash of identity struck between a present and a past, never coupled in the { 124} mind before, is an effect _sui generis_, and not resolvable into any mode or incident of contiguity.
The circumstances of this identifying stroke are so numerous and far-reaching as to demand a special exemplification. Some of the broadest distinctions of intellectual character can be grounded on the distinctive aptitudes of the mind for Contiguity and for Similarity.
Learning, Acquisition, Memory, Habit, all designate the plastic adherence of contiguous impressions. The processes of Classification, Reasoning, Imagination, and the Inventive faculty generally, depend upon the identifying stroke of likeness in unlikeness. Some forms of intellectual strength, as a whole, are best represented by a highly energetic Adhesiveness; distinction as a learner, a follower of routine, turns upon this power. Other, and higher, forms of intelligence depend upon far-reaching strokes of similarity; the identification of likeness shrouded in diversity, expresses much of the genius of the poet, the philosopher, the man of practice.