PROBLEMS OF MODERN SCIENCE 



only a restricted validity, and are applicable with- 

 out modification only to slowly changing or slowly 

 moving systems. There are, in fact, many old- 

 established laws which one now hesitates to employ 

 dogmatically without fear of contradiction. On 

 the other hand, new guiding principles have 

 arisen which were undreamed of only thirty years 

 ago. 



Of these the most far-reaching is the principle 

 of relativity. I am aware that Professor Nicholson 

 has explained to us that it is impossible for a 

 mathematician to explain this principle to a 

 physicist ; a fortiori, therefore, it is more than 

 impossible for a physicist to explain it to a layman. 

 However, I feel that I can speak about what I do 

 not understand, perhaps not as well, but with as 

 much right, as any other kind of philosopher. I 

 shall therefore take the liberty of saying a few 

 words about relativity. In reality there are two 

 not altogether unconnected principles of rela- 

 tivity. The older, which may be regarded as a 

 particular case of the other, is sometimes referred 

 to as the restricted relativity principle. It is 

 based on the empirical fact that it has been found 

 impossible to detect absolute motion by any 

 mechanical, optical, electrical, or other experiments. 

 So many promising experiments in this direction 

 have been tried with negative results that I feel 

 confident that the restricted principle rests on a 

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