

PRIMARY CONCEPTS OF MODERN SCIENCE. 355 



philosophers ! It is like the question which Simon Sachs, in his de- 

 spair, propounded to the gods : * Who will assure us that the star, 

 which the astronomers regard as Uranus, is Uranus in fact ? ' " l 



Physicists generally, however, are in still greater confusion as to 

 the nature of force in another respect. Bodies are said to be endowed 

 with a definite quantity of force, if not with a given number of forces ; 

 it is assumed that to every particular body or particle belongs, or in 

 such body or particle in and by itself is inherent, an invariable meas- 

 ure of energy. This statement, besides involving the confusion just 

 noted as to the reality of force, implies the assumption that force can 

 be an attribute or concomitant of a single body or particle as such. 

 This assumption ignores the fact, which is otherwise well known to 

 physicists, that the very conception of force depends upon the relation 

 between two terms at least. "Every force," says Clerk Maxwell 

 (" Theory of Heat," p. 94), " acts between two bodies or parts of 

 bodies." A " constant central force," therefore, as belonging to an 

 individual atom in and by itself, is an impossibility. 



"We have now arrived at a point where it will be profitable to re- 

 cur for a moment to the proposition of Du Bois-Reymond referred to 

 at the beginning of my first paper, that the whole problem of physical 

 science consists in " the resolution of all changes in the material world 

 into motions of, atoms caused by their constant central forces." The 

 entire passage in Du Bois-Reymond's text, from which I extracted two 

 sentences, reads as follows : " Natural science — more accurately ex- 

 pressed, scientific cognition of Nature, or cognition of the material 

 world by the aid and in the sense of theoretical physical science — is a 

 reduction of the changes in the material world to motions of atoms 

 caused by central forces independent of time, or a resolution of the 

 phenomena of Nature into atomic mechanics. It is a fact of psycho- 

 logical experience that, whenever such a reduction is successfully 

 effected, our craving for causality is, for the nonce, wholly satisfied. 

 The propositions of mechanics are mathematically representable, and 

 carry in themselves the same apodictic certainty which belongs to the 

 propositions of mathematics. When the changes in the material world 

 have been reduced to a constant sum of potential and kinetic energy, 

 inhering in a constant mass of matter, there is nothing left in these 

 changes for explanation. 



" The assertion of Kant in the preface to the ' Metaphysical Rudi- 

 ments of Natural Science,' that 4 in every department of physical sci- 

 ence there is only so much science, properly so called, as there is 

 mathematics,' is to be sharpened by substituting * mechanics of atoms ' 

 for 'mathematics.' This was evidently his own meaning when he 

 denied the name ' science ' to chemistry. It is not a little remarkable 

 that in our time chemistry, since it has been constrained, by the dis- 



1 " Das Sonnensyscem, oder neue Theorie vom Bau der Welten, von Simon Sachs," p. 

 193, C. Fechner. 



