As experience clusters certain ideas into sets, so does the mind too, by its occult nature, cluster certain ideas into sets. These sets have various forms of connection. The simplest are sets of things all on one footing and agreeing in each belonging to the set. Such a set is a class. The clustering of ideas into classes is the simplest form which the association of ideas by the occult nature of ideas, or of the mind, can take. Now, just as in association by contiguity an idea calls up the idea of the set in which experience has placed it, and thence one of the other ideas of that set, so in association by resemblance an idea calls up the idea of the set in which the mind’s occult virtue places it, and that conception perhaps gives, owing to some other circumstance, another of the particular ideas of the same set. Everybody has heard in conversation a person remark, “What you say puts me in mind of a similar occurrence.” That is suggestion by resemblance. Association by contrast is a case of association by resemblance, which is so called after its most prominent variety. Suggestion by resemblance means, let it be repeated, the indirect suggestion by one idea of another which has, by virtue of the occult nature of ideas or of the mind, been associated with it into one set. All the suggestions of pure mathematics, of which there is a vast body, are associations by resemblance.