thought, just as the concept we form to ourselves of matter is but the symbol of some form of power absolutely and forever unknown to us, as the representation of all objective activities in terms of motion is only a symbolic representation, and not a knowledge of them. Symbols of unknown forms of existence, whether in the case of matter, motion, or mind, are mere representations which do not determine anything about the ultimate substance of either. "Our only course is constantly to recognize our symbols as symbols only, and to rest content with that duality of them which our constitution necessitates. The unknowable as manifested to us within the limits of consciousness in the shape of feeling, being no less inscrutable than the unknowable as manifested beyond the limits of consciousness in other shapes, we approach no nearer to understanding the last by rendering it into the first."[117] Discarding, then, the expression "substance of mind," excepting as a mere symbol, Mr. Spencer passes to the "composition of mind"; and here we reach his explanation of mind as an evolution traceable through ascending stages of composition, conformably to the laws of evolution in general, so that the composition of mind, as something evolved out of simple elements, does not need or involve a symbolical representation in the terms of matter. The method of composition, by which the whole fabric of mind is constituted, from the formation of its simplest feelings up to the formation of the complex aggregates of feelings which are its highest developments, can now be sketched. A sensation is formed by the consolidation of successive units of feeling; but the feelings called sensations can not of themselves constitute mind, even when many of different kinds are present together. When, however, each sensation, as it occurs, is linked in association with the faint forms of previous sensations of the same kind, mind is constituted; for, by the consolidation of successive sensations, there is formed a knowledge of the particular sensation as a distinct subject of what we call thought, or the smallest separable portion of thought as distinguished from mere confused sentiency. Thus, as the primitive units of feeling are compounded into sensations, by the same method simple sensations, and the relations among them, are compounded into states of definite consciousness. The next highest stage of mental composition is a repetition of the same process. Take a special object, which produces in us a vivid cluster of related sensations. When these are united with the faint forms of like clusters that have been before produced by such objects, we know the object. Knowledge of it is the assimilation of the combined group of real feelings which it excites, with one or more preceding ideal groups which were once excited by objects of the same kind; and, when the series of ideal groups is large, the knowledge is clear. In the same way, by the connections between each special