PRIMARY CONCEPTS OF MODERN SCIENCE. 95 



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 ab- 

 surdity that I believe no man, who in philosophical matters has a com- 

 petent 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 li Discours" it found expression in the axiom of Barthol- 

 omew Keckermann (" Systema Physicum," Hanau, 1612) : " Omnis al- 

 teratio 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 " pat- 

 ent 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 au- 

 thority 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, be- 

 cause 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 tlfe sun attracting the earth without an in- 

 tervening 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 iden- 

 tical 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," 



