In antiquity and the middle ages, the schoolmaster and the philosopher were one and the same individual. The new philosophy was the first to separate these two departments; perceiving that the communication of truth is a distinct office from its investigation, and that that difference of office in each case necessitates a corresponding difference in the public, that is the proper object of its exercise. Since, moreover, society may be discriminated into two sorts of mind, admitting of being pictured as the childish and the adults, it is evident that the instructor must find his audience more especially in the former, while the investigator of truth must appeal exclusively to the latter. This he must needs do, to whichever of the sciences he ministers; and not only so, but he must more particularly address himself to a small and select portion of this itself selecter class, constitute them the witnesses and judges of his proceedings, and perceive that both his success in philosophy and the acknowledgment of it can only be founded first and foremost on their approbation. As even in jockeyism and prize-fighting, there are "the knowing ones," similar referees are, by the nature of things, required for the flourishing estate of any science; and evidently in proportion as they might be incompetent to such an office, false or imperfect science must be the result. Locke, acting on this instinctive view, communicated to the public certain observations he had made in mental philosophy, and entitled his work, An Essay on the Human Understanding. He properly called it an essay, because a person who simply aims to investigate truth, undertakes to do his best in the way of trial, endeavor, and experiment, in such sort as to make the word essay appropriate to what he does. The word moreover implies that the thing done, though it is the writer's best, is liable to be incomplete, comparatively imperfect, and, indeed, in the more difficult questions of philosophy, as well as in the less advanced stages of philosophising, is sure to be so. Locke accordingly, having had his attention struck with certain phenomena of the human mind, told the public just what he had observed, and nothing else. Among the observations that he thus imparted, was the process through which the mind seems to go in arriving at the sum of its ideas, and especially the points from which it seems to start in this process.