philosophers! It is like the question which Simon Sachs, in his despair, propounded to the gods: 'Who will assure us that the star, which the astronomers regard as Uranus, is Uranus in fact?'"[1]
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 measure 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 recur 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 expressed, 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 psychological 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 Rudiments of Natural Science,' that 'in every department of physical science 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-
- ↑ "Das Sonnensystem, oder neue Theorie vom Bau der Welten, von Simon Sachs," p. 193, C. Fechner.
