THE REAL WORLD 369 



ourselves open to a more dangerous renewal of the same thrust? At 

 best the pictures drawn by our theorists are symbolist sketches for- 

 ever tinged, as Hertz emphasized, by human predisposition to cer- 

 tain modes of symbolization. If then we find an element of sameness 

 in the pictures drawn by successive theories, is this not testimony for 

 the common humanity of the painters, rather than for the faithfulness 

 of their rendition of a common subject? James argues that: 



We plunge forward into the field of fresh experience with the beliefs 

 our ancestors and we have made already; these determine what we 

 notice; what we notice determines what we do; what we do again 

 determines what we experience; so from one thing to another, al- 

 though the stubborn fact remains that there is a sensible flux, what is 

 true of it seems from first to last to be largely a matter of our own 

 creation. 



Now if unchanging aesthetic and cosmologic considerations were de- 

 cisive in our choice of scientific theories, James' position would be 

 very strong. We have seen that in the term of months or years these 

 subjective considerations may indeed be paramount. But irreducibly 

 objective components loom large in the two longer-term criteria of 

 judgment: heuristic power, effective in the period of decades to gen- 

 erations; ancZ simplicity, effective in the period of generations to 

 centuries. 



Heuristic power. Entirely denying that scientific theories have dis- 

 tinctive heuristic power, one may argue that the illusion of such 

 power is merely a secondary manifestation of correlative efficiency 

 that puts us in full command of what is already known. But only the 

 heuristic function of the theory makes the old knowledge relevant to 

 discovery of the new, by revealing in the old a pointer toward the 

 new. Take as an example the conception by Maxwell, in mid-19th 

 century, of a kinetic theory of gases involving a particular distribu- 

 tion of molecular velocities. More than half a century later Maxwell's 

 theory leads Stern, and then others, to devise novel systems yielding 

 (velocity) data of a species entirely different from the simple pneu- 

 matic data accessible to Maxwell. The new data prove precisely con- 

 cordant with predictions drawn from Maxwell's theory. This, and 

 other like successes, seem to demand, and thus to testify to, the func- 

 tion of Maxwell's theory as something more than a clever mnemonic 

 device. 



