Prosecute the popular knowledge about any point far enough, substituting completeness and accuracy for vagueness, and especially giving numerical definiteness in weight, size, and figure, until the little drop of fact has grown into an ocean, and the mere germ has expanded into a structure with complex interconnexion,--and you will have a science. By its point of origin this luminous body of facts is united to the great circle of human knowledge and ignorance. Each special science is a part, which presupposes a total of much lower organisation, but much wider range than itself: each branch of scientific knowledge grows out of the already existent tree of acquaintance with things. But the part very soon assumes an independence of its own, and adopts a hostile or negative attitude towards the general level of unscientific opinion. This process of what we may, from the vulgar point of view, call abnormal development, is repeated irregularly at various points along the surface of ordinary consciousness. At one time it is the celestial movements calling for the science of astronomy: at another the problem of dividing the soil calling for the geometrician. Each of these outgrowths naturally re-acts and modifies the whole range of human knowledge, or what we may call popular science; and thus, while keeping up its own life, it quickens the parent stock with an infusion of new vigour, and raises the general intelligence to a higher level and into a higher element.
The order of the outcome of the sciences in time, therefore, and their connexions with one another, cannot be explained or understood, if we look only to the sciences themselves. We must first of all descend into the depths of natural thought, or of general culture, and trace the lines which unite science with science in that general medium. The systematic interdependence of the sciences must be chiefly sought for in the workings of thought as a whole in its popular phases, and in the action and reaction of that _general_ human thought with the sciences, those definite organisations of knowledge which form sporadically round the _nuclei_ here and there presented in what would superficially be described as the inorganic mass and medium of popular knowledge. Thus, by means of the sciences in their aggregate action, the material of common consciousness is expanded and developed, at least in certain parts, though the expansion may be neither consistent nor systematic. But so long as this work is incomplete, so long, that is to say, as every point in the line of popular knowledge has not received its due elaboration and equal study, the sciences merely succeed each other in a certain imperfect sequence, or exist in juxtaposition: they do not form a total. The whole of scientific knowledge will only be formed, when science shall be as completely rounded and unified, as in its lower sphere and more inadequate element the ordinary consciousness of the world is now.