THE PRINCIPLES OF SCIENCE 179 



cannot understand; and that is why I cannot get the electromagnetic 

 theory. ... I want to understand hght as well as I can without in- 

 troducing things that we understand even less of. 



Even today many still prefer mechanical models: of all models they 

 are the most directly linked with both familiar everyday experience 

 and the events actually observed in the laboratory. But today few in- 

 deed adhere to Kelvin's opinion that only mechanical models are ac- 

 ceptable. Thoroughly justified for so long as mechanics was the one 

 established scientific system, his conception becomes altogether too 

 narrow as soon as there appear other well developed sciences with 

 which we feel enough at ease to frame our models on them. Today, 

 for example, we happily found "field theories" on a model supplied 

 by precisely that classical electrodynamics Kelvin still found unintel- 

 ligible in itself. A century ago men dealt only in mechanical models 

 formulated explicitly. Today we deploy a diversified array of physi- 

 cal models that, in some cases, may figure only implicitly in a theory 

 constituted by little more than a set of equations. However vague, 

 distant, or incomplete may be the analogy— however deeply hidden 

 in the mathematical formalism may be the physical model— always 

 they are indispensable to our grasp of the theory. But, having forged 

 on beyond mechanical models, we now recognize our capacity to 

 pass beyond any predetermined set of "acceptable" models. 



CONGRUITY 



Accepting the principle of intelligibility, we set ourselves to under- 

 stand a world we assume humanly understandable. We may bolster 

 our courage with the assertion that "nature is simple," so that even 

 feeble human reason may comprehend it. The eflFect is the same if we 

 assert the complexity of nature and the vast power of human reason. 

 Actually of course we need, and can, assert neither simplicity nor 

 complexity of world and mind: our basic assumption is the congruity 

 of world and mind. For the earliest scientists the assumption of con- 

 gruity was justifiable only as, for the religious, Pascal's wager might 

 justify faith in his God. Today that assumption is buttressed by a prog- 

 ress of science Einstein holds ample to demonstrate that nature is 

 humanly comprehensible. 



One may say "the eternal mystery of the world is its comprehensi- 

 bility." . . . 



