272 THE EVOLUTION OF SCIENTIFIC THEORIES 



or allay completely the doubts of all who doubt it. And so again we 

 seem to reach an impasse: how can a new theory ever win general 

 acceptance? The answer lies in the deeper secondary effects of its 

 heuristic power. 



The Natural Selection of 

 Scientific Theories 



Polanyi observes that 



A hostile audience may . . . deliberately refuse to entertain novel 

 conceptions such as those of Freud, Eddington, Rhine or Lysenko, 

 precisely because its members fear that once they have accepted this 

 framework they will be led to conclusions which they— rightly or 

 wrongly— abhor. . . . 



We can now see, also, the great difficulty that may arise in the at- 

 tempt to persuade others to accept a new idea in science. ... to the 

 extent to which it represents a new way of reasoning, we cannot con- 

 vince others of it by formal argument, for as long as we argue within 

 their framework, we can never induce them to abandon it. Demon- 

 stration must be supplemented, therefore, by forms of persuasion 

 which can induce a conversion. 



Particularly when cosmologic issues are involved, no analyses of, or 

 arguments from, the criteria we have examined can constitute such 

 persuasion. No demonstration of mathematical elegance could rec- 

 oncile some men to the immensity of the universe depicted by the 

 Copernican theory; no demonstration of correlative efficiency could 

 persuade Agassiz to accept a theory of evolution by natural selec- 

 tion; no demonstration of heuristic power could convert Planck and 

 Einstein to belief in a quantum mechanics that denies all but statisti- 

 cal determinism. Planck, indeed, denies the whole possibility of con- 

 version in such cases. 



An important scientific innovation rarely makes its way by gradually 

 winning over and converting its opponents: it rarely happens that 

 Saul becomes Paul. What does happen is that its opponents gradually 

 die out and that the growing generation is familiarized with the idea 

 from the beginning: . . . 



How does it happen that new opponents are not born to replace 

 those who die? We discern a mechanism of natural selection that 



