ceived theories of physical science respecting the nature of force, it is manifest, irrespective of the considerations which I have presented in this and the preceding essays, that force is not an individual thing or distinct entity which presents itself directly either to observation or to thought, but that, so far as it is taken as a definite and unital term in the operations of thought, it is purely a fiction of the intellect. The cause of motion, or change of motion, in a body is simply the condition or group of conditions upon which this motion depends; and this condition, or group of conditions, as we have already seen, is always a corresponding motion, or change of motion, in the bodies, outside of the moving body, which are its correlates. Otherwise expressed, force is a mere inference from the motion itself under the universal conditions of reality, and its measure, therefore, is simply the effect for which it is postulated as a cause; it has no other existence. The only reality of force and of its action is the correspondence between physical phenomena in conformity to the principle of the essential relativity of all material existence.
That force has no independent reality is so plain and obvious that it has been proposed by some thinkers to abolish the term force, like the term cause, altogether. However desirable this might be in some respects, it is impossible, for the reason that the concept "force," when properly interpreted in terms of experience, is valid, and, if its name were abolished, it would instantly reappear under another name. There is hardly any concept which has not, in science as well as in metaphysics, given rise to the same confusion which prevails in regard to "force" and "cause;" and the blow leveled at these would demolish all concepts whatever. Nevertheless, it is of the greatest moment, in all speculations concerning the interdependence of physical phenomena, never to lose sight of the fact that the reality of force is purely conceptual, and that it is not a distinct and individual tangible or intangible entity.
How imperfectly all this is understood by the physicists of our time appears at once upon an examination of elementary treatises as well as original disquisitions on physical science. Thus the relation of force to mechanical motion is constantly spoken of as a "fact" ascertained by experience and verified experimentally beyond the possibility of question. In a learned article by J. Croll, published in the July number, 1872, of the Philosophical Magazine ("What determines Molecular Motion," etc., Phil. Mag., fourth series, vol. xl., p. 37), it is said: "In regard to the first question (what produces motion) there is no diversity of opinion. All agree that what produces change or causes motion is force." The obvious meaning of this is that it might possibly admit of question whether material change or motion is produced by force or something else, and that physicists, on the whole, have come to the conclusion that it is produced by force. Such a question ought, indeed, to be solemnly pondered by grave