SCIENCE, COMMON SENSE, AND DECENCY 7 



volving as they do natural selection acting upon mutations, must depend 

 at almost every stage upon phenomena which originate in single atoms. 



An idea that develops in a human brain seems to have all the character- 

 istics of divergent phenomena. All through our lives we are confronted 

 with situations where we must make a choice and this choice may some- 

 times alter the whole future course of our lives. Occasionally such deci- 

 sions are made by tossing a coin — an event which seems unpredictable — 

 but there is no reason to doubt that single atomic processes, or small 

 groups of them which follow statistical laws, may often be a determining 

 element. 



When certain bacteria are heated until they begin to die it is found that 

 in successive intervals of time the same fraction of the survivors succumb. 

 This seems to prove that the life of these cells depends on a single unstable 

 molecule whose change involves death. It is thus a matter of pure chance 

 as to which particular individuals die within a given period. 



The examples that I have given of convergent and of divergent phe- 

 nomena are purposely chosen as extreme types. Actually there are many 

 intermediate cases that are not clearly one or the other. Or, again, phe- 

 nomena which start out to be divergent may apparently degenerate into 

 the convergent type. For example, if the photograph of the Wilson cloud 

 track did not prove to be of interest it would soon be forgotten and might 

 have no further ascertainable effects on human lives. Similarly in the 

 evolutionary process a new species that originates as a single divergent 

 phenomenon may not survive and its effect soon seems to die out. 



We must recognize, nevertheless, that a divergent phenomenon which 

 once occurs permanently alters details of molecular arrangements in the 

 convergent systems that follow it and thus conditions may be brought 

 about which favor the occurrence of new divergent phenomena. In a world 

 in which divergent or quantum phenomena occur we can thus have no 

 absolute relation of cause and effect. 



As the implications of the uncertainty principle, especially as applied 

 to divergent phenomena, are more generally recognized the limitation of 

 the idea of causality should have profound effects on our habits of thought. 

 The science of logic itself is involved in these changes. Two of the funda- 

 mental postulates of logic are known as the law of uniformity of nature 

 and the law of the excluded middle. The first of these laws is equivalent to 

 the postulate of causality in nature. The second law is simply the familiar 

 postulate that a given proposition must be either true or false. In the past 

 these so-called laws have formed the basis of much of our reasoning. It 

 seems to me, however, that they play no important part in the progress of 

 modern science. The cause and effect postulate is only applicable to con- 

 vergent phenomena. The second postulate in assuming that any proposi- 



