Jandart 18, 1918] 



SCIENCE 



57 



showed that the description was simpler 

 and briefer if expressed in terms of the 

 second differential coefiBcients instead of 

 the first. Similar ideas have been devel- 

 oped with greater generality by Mach, who 

 finds the final purpose of scientific theories 

 to be economy of thought, and classes the 

 search for causes, explanations and dy- 

 namical theories, among the metaphysical 

 prejudices which hinder the progress of 

 science. 



The criticism of Kirchhoff and IMach is 

 logical and convincing. No unprejudiced 

 person can doubt that, after a discovery is 

 made, it may be interpreted in their way 

 and that, on the whole, this interpretation 

 is the cleanest, most rational and most free 

 from human weakness. But from the 

 pragmatic point of view, and in the light 

 of experience of the course of science in the 

 past, it may well be doubted if their at- 

 titude of mind is a useful one in the work 

 of investigation and discovery as disting- 

 uished from subsequent criticism and clari- 

 fication. A somewhat extreme example of 

 militant advocacy of the descriptive method 

 was furnished about twenty years ago by 

 the school of energetics under the leader- 

 ship of Ostwald. Any use of atomic hy- 

 potheses was by them regarded as evidence 

 of feebleness of intellect and slavery to 

 metaphysical prejudices. Their opinions 

 were based upon an incomplete acquaint- 

 ance with the state of physical knowledge 

 even at that time; they were vigorously 

 opposed in numerous papers by Boltz- 

 mann who demonstrated "the indispensa- 

 bility of atomistics in natural philosophy" 

 in a most convincing manner. As we all 

 know, the progress of experimental dis- 

 covery has long since convinced the ener- 

 geticians that no adequate description of 

 material phenomena can be given without 

 the use of atomic theories. 



Boltzmann has also pointed out^ that even 

 the most elaborate and detailed mechanical 

 theories of Kelvin or Maxwell, for example, 

 are regarded by their authors themselves 

 merely as models; that description by 

 means of models, if accurate and conven- 

 ient, is quite as legitimate as description 

 by means of differential equations; and 

 that the method could be thus amply justi- 

 fied even on the most sophisticated philo- 

 sophical principles. 



It may, I think, safely be said that the 

 most remarkable example in physical sci- 

 ence of the purely descriptive theory — the 

 one with the least taint of the fallacy of 

 cause and effect — is Einstein's theory of 

 relativity. All of us who studied our Jlax- 

 well in the early nineties or previous to 

 that time, and who have kept an interested 

 eye upon the progress of electrodynamics 

 in the intervening years, are aware of the 

 great difficulties which were encountered 

 in the attempt to extend the Maxwellian 

 electrodynamics to moving bodies. Max- 

 well and Hertz both went astray in that 

 portion of the subject. "We all remember 

 how these difficulties were slowly cleared 

 up, step by step, especially by the masterly 

 work of Lorentz, but with important con- 

 tributions by J. J. Thomson, Heaviside, 

 Larmor, FitzGerald, Max Abraham and 

 others. What we now call the electron 

 theory had its origin in this attack upon 

 the electrodynamics of moving matter, and 

 was not the result of any prevision that 

 within a few years we should be able to 

 handle, and experiment with, the disem- 

 bodied electrons themselves. The final 

 puzzle was the reconciliation of the result 

 of the Michelson-Morley experiment with 

 the facts of aberration, the Fresnel "co- 

 efficient of entrainment" and other optical 

 knowledge. I\Iost of us can remember the 

 great perplexity which this caused; and it 



5"Populure Schriften," p. 1. 



