i8 THE LIMITATIONS OF SCIENCE 



more of molecules, ions, and the ether, than that they 

 may exist; but ponderable matter, as perceived by the 

 senses, has an objective existence, or else there is no 

 place for science. Since Kant's time the existence or 

 the non-existence of those insensible links in the uni- 

 versal machine is known to be equally demonstrable; 

 we have no criterion of proof. It is curious that we 

 still refuse to acknowledge this. If we look askant 

 at Kant, the metaphysician, we have only to turn to 

 Lagrange, whose scientific claims cannot be ignored, 

 and find he has proved by rigid mathematical analysis 

 that any phenomenon, which obeys the law of conser- 

 vation of energy, is capable of an explanation by a 

 mechanical theory ; but, and here is the important point, 

 as there is always one adequate theory, so there are 

 also an indefinite number of other mechanical theories 

 which will, so far as our minds are concerned, satisfy 

 all the requirements of the case. We have no criterion 

 in mechanics by which we may determine what is the 

 actual process of nature. There is no experimenium 

 crucis, and we choose the explanation which for the 

 moment seems to be the simplest. 



Our inability to decide unequivocally for one me- 

 chanical hypothesis instead of another is shown also 

 by the actual history of physical science. Since the 

 time of Huygens and Newton we have attempted to 

 settle the question whether light is due to a wave 

 motion in an ether or to small particles emitted from 



