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the mediation of something else, which is not material, operate upon and affect other matter without mutual contact. . . . That gravity should be innate, inherent, and essential to matter, so that one body may act on another, at a distance, through a vacuum, without the mediation of any thing else by and through which their action and force may be conveyed from one to the other, is to me so great an absurdity that I believe no man, who in philosophical matters has a competent faculty of thinking, can ever fall into it."

The thesis of the impossibility of actio in distans has been a standing dogma among physicists ever since the revival of physical science, three centuries ago. Twenty-five years before the publication of Descartes's "Discours" it found expression in the axiom of Bartholomew Keckermann ("Systema Physicum," Hanau, 1612): "Omnis alteratio fit per contactum;" and Descartes's whole physical system had its root in the proposition that "a body can no more act where it is not than it can act when it has ceased to be, the principle, cessante causa cessat effectus, holding good in either case." It was this "patent absurdity" of material action through empty space which led the greatest mathematicians of Continental Europe (the elder Bernouillis, Huyghens, etc.) to reject the doctrines of Newton's "Principia," and induced Leibnitz to construct his system of "cosmic circulations," in which the old Cartesian vortices reappeared in a new dress, and under another name.

The conflict between the theory of distant attraction and the authority of the "scientific imagination" is one of the instances adduced by John Stuart Mill ("System of Logic," book ii., chap, v., §6) in support of his proposition that conceivability is no test of truth, because it is the simple result of familiarity of thought or experience; and he expresses the opinion that "the majority of scientific men have at last learned to conceive the sun attracting the earth without an intervening fluid, and that "the ancient maxim, that a thing cannot act where it is not, probably is not now believed by any educated person in Europe" ("Logic," book v., chap, iii., §3). But Herbert Spencer ("Principles of Psychology," ii., 409) justly doubts the truth of this opinion, expressing the belief that "the most that can be said is that they" (the scientific men) "have given up attempting to conceive how gravitation results." How formidable the difficulty under discussion still appears to the minds of physicists at the present day, and how completely the mental predicament of the nineteenth century is identical with that of the seventeenth, is evident from the many recent renewals of the attempt to construe the action of gravity as a case of ethereal pressure or impact. I content myself with the citation of a very characteristic paragraph, written long after the sentences quoted from Mill, in a late article of Prof. Challis "On the Fundamental Ideas of Matter and Force in Theoretical Physics" (Philosophical Magazine, 4, vol. xxxi., p. 467). "There is no other kind of force,"