Psychologists recognize that the suggestion of one idea by another may take place according to either one of two different principles; for an idea may suggest another like it, or it may suggest another which has been connected with it in experience. Thus, the thought of Niagara may suggest a hero or anything else that is grand, and so similar to the cataract, or it may suggest a crowd of importunate hackdrivers, which is connected with the place in every visitor’s experience.
Association of the latter kind, association by contiguity as it is called, is the more typical. In it the characteristics of mental association are more strongly marked. Association by similarity is related to association by contiguity somewhat as our inward consciousness is related to outward experience; the one association is due to a connection in outward experience, the other to a connection in our feelings.