The external world of our Experience seems to be composed of sensible impressions. The ever present visual panorama combined with the constant occurrence of other sensations suggests that Nature is, as has so often been asserted, simply another name for the sensible presentation. A truer view of Nature was adumbrated by Aristotle when he formulated the theory of an Energy ever generative of the sensible. If the founders of Science did not fully grasp the Aristotelian conception, it is at least certain that they looked upon Nature not merely as a sensible presentation but as a process--a dynamic operation. It was to the study of these operations, to the measurement of the natural forces or normal categories of physical action that Galileo and Newton devoted themselves. The true estimate of a moving force may indeed be said to have been their first great problem, just as the law of universal gravitation was their grandest generalisation. It was to this sure instinct that the founders of Science owed their success. Had they devoted themselves to the mere study of sensations--of blue things and green things, of hard things and soft things, of loud things and silent things--Science as an efficient and co-ordinated system would never have come into being. * * * * * Having struck the right path, they moved rapidly along it, leaving the Schoolmen and Philosophers behind them, suspicious, hostile, and amazed. But Philosophy did not remain altogether negative. The new movement extended itself to Metaphysics, and under the leadership of Descartes a resolute effort was made to reform Philosophy on sympathetic lines. It was in the true spirit of Socrates that Descartes advanced his famous method of Doubt. The whole fabric of beliefs and rational principles was to be subjected to a re-examination, and Descartes found himself on bedrock when he touched his famous _Cogito, ergo sum._ The simple fact or act of Doubt implied the Activity--the Reality therefore--of the Doubter. But the cogitant subject was reduced very much to the condition of a _tabula rasa_, and when Descartes proceeded to fill up the blank with a rediscovery on more scientific lines of the essentials of Cognition he found his basal feature in Extension. Tridimensional Space seemed the simple elementary framework of our Knowledge of Nature. The method of Descartes was further extended by the English philosopher Locke. Those qualities which formed the elements of Knowledge were described by him as the primary qualities of body; the sensible presentation comprised also the secondary qualities which seemed to be in some way superposed upon and contained within the former. * * * * *