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Rh considered as resisting the communication of motion." As is readily seen, all these definitions imply, nevertheless, that matter can be moved or changed only from without, by forces external to matter itself. Newton expressly ("Principia," Definitio IV.) speaks of force as "impressed upon a body, and as exerted upon it to change its state of rest or uniform motion in a straight line."

There is little difficulty in understanding how this language, in connection with the etymological import of the word "inertia," led to the assumption that matter is essentially passive, or, as it is commonly expressed, dead. There are other reasons for this assumption, connected with the evolution, not only of scientific concepts, properly so called, but of cosmological ideas, to which I shall have occasion, perhaps, to recur in the sequel; indeed, Newton's definitions which I have just cited are simply instances of the intellectual postulates of his time. And the mathematical treatment of mechanical problems, from the nature of its methods, necessitates the fiction that force and mass are separate and distinct terms. In general, it may be said that the assumption of the absolute passivity of matter is one of those errors which are inevitable in the progress of knowledge—one of the "clay moulds in which the bells of scientific truth are cast." But the perpetuation of this error is one of the most fatal impediments to real scientific progress in our day, and is fruitful of vagaries which are wholly incommensurable with the real state of modern scientific knowledge. Thus, Prof. Philip Spiller, the author of a very serviceable manual of physics, and a prolific writer on scientific subjects, has recently published a cosmological treatise, whose theorems are founded upon the express proposition (op. cit., p. 4) that "no material constituent of a body, no atom, is in itself originally endowed with force, but that every such atom is absolutely dead, and without any inherent power to act at a distance." It appears from the further contents of Spiller's treatise, that he not only denies force to the atoms taken singly, but that he also denies the possibility of their mutual action. He is driven, therefore, to the assertion of the independent substantiality of force; and, accordingly, he assumes force to be an all-pervading quasi-material presence—as he terms it, an "incorporeal matter" (unkoerperlicher Stoff). In utter disregard of the fundamental correlation of force and mass, Spiller identifies his force-substance with the ordinary luminiferous ether, so that this phantom, which, in the view of other physicists, is not only imponderable, but destitute of cohesive, chemical, thermal, electric, and magnetic forces (which, indeed, must be destitute of them if it is to serve as the mere substratum of these various modes of motion)—which therefore is, if possible, still more "dead" than ordinary ponderable matter—now suddenly, without changing its name, and without ceasing to be the substratum of luminar and other undulations, comes to be the very quintessence of all possible energy.