210 THE PRINCIPLES OF SCIENCE 



"natural" established by one or a very few items in the foundation 

 le\'el of substantive principle may predetermine the general direc- 

 tion, and the success or failure, of the scientific endeavor of entire 

 generations. For example, consider how for millennia men wrestled 

 unsuccessfully with the problem of motion. That problem is com- 

 pletely restructured with the coming of a new substantive principle: 

 the Newtonian concept of inertia. Continued uniform rectilinear mo- 

 tion, something never observed in nature, is now considered "natu- 

 ral" and in need of no explanation. Only changes of motion (some of 

 which had earlier seemed entirely "natural") now qualify as eflFects 

 for which causes will be sought. One then no longer asks how it is that 

 a projectile is maintained in motion— a query that proved unanswer- 

 able. Instead one asks how it happens that a projectile moves not in a 

 straight line but along a curved path— a query that proves satisfyingly 

 answerable. 



What are for us the ultimate substantive principles? I do not know. 

 Much must depend on the problem at issue. For terrestrial phenom- 

 ena on the molar scale presumably Euclidean geometry, Newtonian 

 mechanics, and the various conservation laws will represent substan- 

 tive principles as ultimate as any. Are the conservation laws really 

 substantive principles? Consider that, as earlier noted, we simply 

 multiply species of energy as required to find continuity in change. 

 The strength of that determination then itself lends almost magic 

 power to the concept "energy," of which Poincare remarks : 



. . . the word "energy" . . . made the law by eliminating the ex- 

 ceptions, since it gave the same name to things differing in matter 

 and like in form. 



Does the final conservation law then represent anything more than 

 an expression of the determination with which we set out? Consider 

 too that, though today the conservation of mass (or mass-energy) 

 may seem no more than an exhaustively confirmed colligative rela- 

 tion asserted as a principle, it was pronounced as a principle by 

 Lavoisier (among others) well in advance of that confirmation. 



One can lay down as a principle that in every operation there is an 

 equal quantity of matter before and after the operation. 



Are all conservation laws simply expressions of our determination to 

 find continuity, and thus simply human conventions? Are all sub- 

 stantive principles simply conventions? 



