226 THE LIMITATIONS OF SCIENCE 



lates as the conservation of matter and energy, or they 

 are advanced as tentative laws when a certain number 

 of phenomena can conveniently be classified; they be- 

 come steadily more exact as our knowledge of the sub- 

 ject increases and are finally accepted as laws; such 

 has been the history of the second law of thermo- 

 dynamics, of the law of evolution, and numerous 

 others. The difference between these proper hypoth- 

 eses or laws and other hypotheses is that a law does 

 not attempt to explain the mechanism of nature as an 

 hypothesis does. That is, laws may be readily recog- 

 nized because they deal only with sensible matter and 

 its attributes and can thus be subjected to a rigid 

 test of their truth ; all other hypotheses, since they at- 

 tempt to explain natural actions, must create fictitious 

 substances and attributes and when they are found to 

 depart from facts they are not discarded but are merely 

 modified by arbitrarily altering the fictitious substances 

 on which they depend. This difference is shown clearly 

 by Darwin's law of evolution and his hypotheses of 

 natural selection and pangenesis, or by Newton's law 

 of gravitation and his hypothesis of light corpuscles. 

 The history of science points to no more certain conclu- 

 sion than that laws persist and hypotheses decay. For 

 this reason, I have emphasized the distinction between 

 the two and have limited the term, hypothesis, to those 

 scientific theories which require the creation of fictitious 

 substances and occult forces and whose use has caused 



