Common sense appears thus as a perfectly definite stage in our understanding of things, a stage that satisfies in an extraordinarily successful way the purposes for which we think. 'Things' do exist, even when we do not see them. Their 'kinds' also exist. Their 'qualities' are what they act by, and are what we act on; and these also exist. These lamps shed their quality of light on every object in this room. We intercept IT on its way whenever we hold up an opaque screen. It is the very sound that my lips emit that travels into your ears. It is the sensible heat of the fire that migrates into the water in which we boil an egg; and we can change the heat into coolness by dropping in a lump of ice. At this stage of philosophy all non-European men without exception have remained. It suffices for all the necessary practical ends of life; and, among our own race even, it is only the highly sophisticated specimens, the minds debauched by learning, as Berkeley calls them, who have ever even suspected common sense of not being absolutely true. But when we look back, and speculate as to how the common-sense categories may have achieved their wonderful supremacy, no reason appears why it may not have been by a process just like that by which the conceptions due to Democritus, Berkeley, or Darwin, achieved their similar triumphs in more recent times. In other words, they may have been successfully DISCOVERED by prehistoric geniuses whose names the night of antiquity has covered up; they may have been verified by the immediate facts of experience which they first fitted; and then from fact to fact and from man to man they may have SPREAD, until all language rested on them and we are now incapable of thinking naturally in any other terms. Such a view would only follow the rule that has proved elsewhere so fertile, of assuming the vast and remote to conform to the laws of formation that we can observe at work in the small and near. For all utilitarian practical purposes these conceptions amply suffice; but that they began at special points of discovery and only gradually spread from one thing to another, seems proved by the exceedingly dubious limits of their application to-day. We assume for certain purposes one 'objective' Time that AEQUABILITER FLUIT, but we don't livingly believe in or realize any such equally-flowing time. 'Space' is a less vague notion; but 'things,' what are they? Is a constellation properly a thing? or an army? or is an ENS RATIONIS such as space or justice a thing? Is a knife whose handle and blade are changed the 'same'? Is the 'changeling,' whom Locke so seriously discusses, of the human 'kind'? Is 'telepathy' a 'fancy' or a 'fact'? The moment you pass beyond the practical use of these categories (a use usually suggested sufficiently by the circumstances of the special case) to a merely curious or speculative way of thinking, you find it impossible to say within just what limits of fact any one of them shall apply.