When any two psychical real entities, any two ideaswhether perceptions, remembrances, or factitious mental images-have, either at the same instant, or at two contiguous or nearly contiguous instants, been present to the mind, each of them (such is the effect of this conjunction, however transient) acquires in the event of its reappearance a tendency to draw forth and introduce the other ; and the more frequently the conjunct appearance is repeated the more apt, cceteris paribus, is this tendency or probability to ripen into actuality.