If our interests were single and uniform, one consistent scheme of intellectual knowledge would suffice. We need never be in fundamental contradiction with ourselves. Every advance in knowledge would illustrate and confirm what we had already learned. But we are not of this simple constitution. We are first and essentially minds, we are next and temporarily embodied minds, and in each of these characters we have distinct and, to a great extent, conflicting interests. Hence we have to acquire different species of knowledge and admit different standards of truth. The ideas that serve the interests of the embodied man are false to the same man considered apart from his embodiment, and contrariwise--false, in the sense of being useless and perhaps misleading. Hence the existence of Common-sense for the embodied interests, and Philosophy for the purely mental interests. Science is common knowledge carried to its utmost perfection, but not partaking in the least of the philosophical character. _Realism_ is the notion of perception that is acquired with our common knowledge. It is seldom explicitly defined or defended, for in order to this a comparison with philosophic theories would have to be made, and the defects of realism would be apparent. The realistic view is so named by philosophers to distinguish it from their own views. For corporeal purposes it is useful to believe, and it is therefore relatively true, that there is a real space which would exist although all objects were removed from it. Objects are real solid things stored in space like casks in a cellar. They have fixed dimensions notwithstanding that they appear to contract and dilate as we leave or approach them. It is quite 'natural' they should appear smaller at a distance. Distant perception is conceivable, therefore it is possible, and since calculations based on this assumption are verified by experience, it must and does take place. Time also is as real as space, and would exist by itself though space and its contents were annihilated. It is a sort of stream. All these propositions are true for certain necessary purposes. We begin to form such ideas from the moment we are born, and during the years of infancy we are doing nothing else intellectually but working out the notions of space, time, magnitude, distance. Most of our school education is of the same kind. By the time we reach maturity realism has become so rooted in our intellect that--as regards the majority of men--no sceptical considerations are strong enough to unsettle them. For why? They enable the natural man to provide sufficiently well for his bodily needs and other needs depending therefrom, and he has therefore no motive for doubting his realism or for acquiring any other sort of ideas. He is quite right to abide by those which have answered his purposes.