We now approach what is at once a matter of refined analysis, and a set of questions which are of fundamental importance to the whole superstructure of the present work. I mean the nature of abstraction, and the classification of ideas. No small amount of ambiguity still hangs about these important subjects, and in treating of them it is impossible to employ terms the meanings of which are agreed upon by all psychologists. But I will carefully define the meanings which I attach to these terms myself, and which I think are the meanings that they ought to bear. Moreover, I will end by adopting a classification which is to some extent novel, and by fully giving my reasons for so doing. Psychologists are agreed that what they call particular ideas, or ideas of particular objects, are of the nature of mental images, or memories of such objects—as when the sound of a friend’s voice brings before my mind the idea of that particular man. Psychologists are further agreed that what they term general ideas arise out of an assemblage of particular ideas, as when from my repeated observation of numerous individual men I form the idea of Man, or of an abstract being who comprises the resemblances between all these individual men, without regard to their individual differences. Hence, particular ideas answer to percepts, while general ideas answer to concepts: an individual preception (or its repetition) gives rise to its mnemonic equivalent as a particular idea; while a group of similar, though not altogether similar perceptions, gives rise to its mnemonic equivalent as a conception, which, therefore, is but another name for a general idea, thus _generated_ by an assemblage of particular ideas. Just as Mr. Galton’s method of superimposing on the same sensitive plate a number of individual images gives rise to a blended photograph, wherein each of the individual constituents is partially and proportionally represented; so in the sensitive tablet of memory, numerous images of previous perceptions are fused together into a single conception, which then stands as a composite picture, or class-representation, of these its constituent images. Moreover, in the case of a sensitive plate it is only those particular images which present more or less numerous points of resemblance that admit of being thus blended into a distinct photograph; and so in the case of the mind, it is only those particular ideas which admit of being run together in a class that can go to constitute a clear concept.[15]