The various recurrences of a sensation must be recognised as recurrences, and this implies the collection of sensations into classes of similars and the apperception of a common nature in several data. Now the more frequent a perception is the harder it will be to discriminate in memory its past occurrences from one another, and yet the more readily will its present recurrence be recognised as familiar. The perception in sense will consequently be received as a repetition not of any single earlier sensation but of a familiar and generic experience. This experience, a spontaneous reconstruction based on all previous sensations of that kind, will be the one habitual idea with which recurring sensations will be henceforth identified. Such a living concretion of similars succeeding one another in time, is the idea of a nature or quality, the universal falsely supposed to be an abstraction from physical objects, which in truth are conceived by putting together these very ideas into a spatial and permanent system.
Here we have, if I am not mistaken, the origin of the two terms most prominent in human knowledge, ideas and things. Two methods of conception divide our attention in common life; science and philosophy develop both, although often with an unjustifiable bias in favour of one or the other. They are nothing but the old principles of Aristotelian psychology, association by similarity and association by contiguity. Only now, after logicians have exhausted their ingenuity in criticising them and psychologists in applying them, we may go back of the traditional position and apply the ancient principles at a deeper stage of mental life.
Association by similarity is a fusion of impressions merging what is common in them, interchanging what is peculiar, and cancelling in the end what is incompatible; so that any excitement reaching that centre revives one generic reaction which yields the idea. These concrete generalities are actual feelings, the first terms in mental discourse, the first distinguishable particulars in knowledge, and the first bearers of names. Intellectual dominion of the conscious stream begins with the act of recognising these pervasive entities, which having character and ideal permanence can furnish common points of reference for different moments of discourse. Save for ideas no perception could have significance, or acquire that indicative force which we call knowledge. For it would refer to nothing to which another perception might also have referred; and so long as perceptions have no common reference, so long as successive moments do not enrich by their contributions the same object of thought, evidently experience, in the pregnant sense of the word, is impossible. No fund of valid ideas, no wisdom, could in that case be acquired by living.