146 THE LIMITATIONS OF SCIENCE 



into all equations quantitatively and as we can have 

 no conception of an absolute quantity, we have by 

 statute fixed upon certain arbitrary standards of quan- 

 tity, such as the centimeter and foot for length, the 

 gramme and pound for mass, and the second for time. 

 From these prime standards, all other quantities of a 

 mechanical nature may be readily derived, as for ex- 

 ample : velocity is the quotient of a given length by a 

 time; momentum is the product of a mass and a 

 velocity ; energy, one-half the product of a mass and a 

 square of a velocity, etc. 



As I have already pointed out, the science of me- 

 chanics is the only branch of physics which has a 

 completely developed theory. Not only is this the case, 

 but the other branches of physics have been developed 

 from a mechanical basis, in so far at least as the meas- 

 urement of all phenomena is now made in terms of 

 these mechanical units. This coordination in physics 

 has the great advantage of making it possible to express 

 what are apparently unrelated phenomena by using 

 combinations of only three standards of measurement 

 and by a few general equations of mechanics. And 

 we attain the additional advantage of expressing all 

 these phenomena in the most concrete form imagi- 

 nable. But, on the other hand, this method produces 

 a serious break between physics and certain other sci- 

 ences, especially those which rely more exclusively on 

 the sense perceptions as criteria for classification of 



