62 Prof. Max Planck : New Paths 



readily get the impression that theoretical investigation has 

 been rather confused by so many new experimental dis- 

 coveries, which have been for the most part entirely 

 unexpected, and that it is now in a profitless period of blind 

 groping, which contrasts strongly with the clear calm and 

 security marking the theoretical epoch just passed, — an epoch 

 which, with some justification, can be described as the 

 Classical Epoch. Everywhere old and firmly-rooted con- 

 ceptions are attacked, universally recognized theories are 

 discarded and are replaced by new hypotheses, — some of them 

 of a boldness which makes almost intolerable demands on the 

 intelligence even of the scientifically educated, and, in any 

 case, does not appear calculated to strengthen confidence in 

 a steady and effective progress of Science. So the present 

 science of Theoretical Physics may give the impression of 

 an edifice, venerable indeed but fragile, in which one part 

 after another commences to crumble away, and whose 

 foundations even are threatened. 



Yet nothing would be more incorrect than such an idea. 

 It is true that in the main structure of Physical Theories 

 fundamental changes are taking place. But on closer in- 

 spection, we see that this is not a work of destruction but a 

 work of completion and amplification, that certain blocks 

 of the edifice are only removed from their place in order to 

 find a better and securer place elsewhere, and that the real 

 foundations of theory are still as firm and safe as they have 

 ever been. I shall endeavour to prove in some detail this 

 assertion. 



First a general consideration : — The initial impulse to- 

 wards revision and modification of a physical theory comes 

 almost aways from the discovery of one or more facts which 

 do not fit into the present framework of the theory. Facts 

 always furnish the Archimedian point from which even the 

 most ponderous theory may be lifted off its hinges. In that 

 sense nothing is more interesting to the real Theoretician 

 than a fact which directly contradicts a hitherto universally 

 accepted theory, for it is just here that his real work 

 begins. 



How what are we to do in a case like this ? Only one 

 thing is certain ; something must be changed in the accepted 

 theory, and in such a manner that it agrees with the new 

 fact, but it is often a difficult and complicated question at 

 what point of the theory the correction is to be applied, for 

 one fact is insufficient to furnish a theory. A theory, indeed, 

 consists as a rule of a whole series of theorems connected 

 with each other. It may be compared to a complicated 



