I] OF NEWTONIAN PHYSICS 21 



steeped to the marrow (as Henri Poincare once said) in the old laws, 

 and in no danger of forgetting them, may we be allowed to learn 

 how they have their remote but subtle limitations, and cease afar 

 off to be more than approximately true *. Kant's axiom of causahty, 

 that it is denknotwendig — indispensable for thought — remains true 

 however physical science may change. His later aphorism, that all 

 changes take place subject to the law which links cause and effect 

 together — "alle Veranderungen geschehen nach dem Gesetz der 

 Verkniipfung von Ursache und Wirkung" — is still an axiom a priori, 

 independent of experience: for experience itself depends upon its 

 truth t- 



* So Max Planck himself says somewhere: "In my opinion the teaching of 

 mechanics will still have to begin with Newtonian force, just as optics begins in 

 the sensation of colour and thermodynamics with the sensation of warmth, 

 despite th^ fact that a more precise basis is substituted later on." 



t "Weil er [der Grundsatz das Kausalverhaltnisses] selbst der grund der Moglich- 

 keit einer solchen Erfahrung ist": Kritik d. reinen Vernunft, ed. Odicke, 1889, p. 221. 

 Cf. also G. W. Kellner, Die Kausalitat in der Physik, Ztschr.f. Physik, lx,iv, pp. 568- 

 580. 1930. 



