These various partial views or abstractions, by means of which the mind looks out upon its object, are called sciences, and embrace respectively larger or smaller portions of the field of knowledge; sometimes extending far and wide, but superficially, sometimes with exactness over particular departments, sometimes occupied together on one and the same portion, sometimes holding one part in common, and then ranging on this side or that in absolute divergence one from the other. Thus Optics has for its subject the whole visible creation, so far forth as it is simply visible; Mental Philosophy has a narrower province, but a richer one. Astronomy, plane and physical, each has the same subject-matter, but views it or treats it differently; lastly, Geology and Comparative Anatomy have subject-matters partly the same, partly distinct. Now these views or sciences, as being abstractions, have far more to do with the relations of things than with things themselves. They tell us what things are, only or principally by telling us their relations, or assigning predicates to subjects; and therefore they never tell us all that can be said about a thing, even when they tell something, nor do they bring it before us, as the senses do. They arrange and classify facts; they reduce separate phenomena under a common law; they trace effects to a cause. Thus they serve to transfer our knowledge from the custody of memory to the surer and more abiding protection of philosophy, thereby providing both for its spread and its advance:—for, inasmuch as sciences are forms of knowledge, they enable the intellect to master and increase it; and, inasmuch as they are instruments, to communicate it readily to others. Still, after all, they proceed on the principle of a division of labour, even though that division is an abstraction, not a literal separation into parts; and, as the maker of a bridle or an epaulet has not, on that account, any idea of the science of tactics or strategy, so in a parallel way, it is not every science which equally, nor any one which fully, enlightens the mind in the knowledge of things, as they are, or brings home to it the external object on which it wishes to gaze. Thus they differ in importance; and according to their importance will be their influence, not only on the mass of knowledge to which they all converge and contribute, but on each other.