THE LAW OF SUBSTANCE. 219 



accident of the date of their respective discoveries. 

 The earlier and more accessible chemical law of the 

 persistence of matter was detected by Lavoisier in 

 1879, and, after a general application of the balance, 

 became the basis of exact chemistry. On the other 

 hand, the more recondite law of the persistence of 

 force was only discovered by Mayer in 1842, and only 

 laid down as the basis of exact physics by Helmholtz. 

 The unity of the two laws — still much disputed — is 

 expressed by many scientists who are convinced of it 

 in the formula : " Law of the persistence of matter 

 and force." In order to have a briefer and more 

 convenient expression for this fundamental thought, I 

 proposed some time ago to call it the " law of 

 substance" or the "fundamental cosmic law"; it 

 might also be called the " universal law," or the 

 " law of constancy," or the " axiom of the constancy 

 of the universe." In the ultimate analysis it is found 

 to be a necessary consequence of the principle of 

 causality. 1 



The first thinker to introduce the purely monistic 

 conception of substance into science and appreciate 

 its profound importance was the great philosopher 

 Baruch Spinoza ; his chief work appeared shortly 

 after his premature death in 1677, just one hundred 

 years before Lavoisier gave empirical proof of the 

 constancy of matter by means of the chemist's 

 principal instrument, the balance. In his stately 

 pantheistic system the notion of the world (the 

 universe, or the cosmos) is identical with the all- 

 pervading notion of God ; it is at one and the same 

 time the purest and most rational monism and the 



1 Cf. Monism, by Ernst Haeckel. 



