708 SCIENCE AND HYPOTHESIS 



to all appearance widely separated. I venture to say that these theo- 

 ries are all simultaneously true ; not merely because they express a true 

 relation that between absorption and abnormal dispersion. In the 

 premisses of these theories the part that is true is the part common 

 to all: it is the affirmation of this or that relation between certain 

 things, which some call by one name and some by another. 



The kinetic theory of gases has given rise to many objections, to 

 which it would be difficult to find an answer were it claimed that the 

 theory is absolutely true. But all these objections do not alter the 

 fact that it has been useful, particularly in revealing to us one true 

 relation which would otherwise have remained profoundly hidden 

 the relation between gaseous and osmotic pressures. In this sense, 

 then, it may be said to be true. 



When a physicist finds a contradiction between two theories which 

 are equally dear to him, he sometimes says : " Let us not be troubled, 

 but let us hold fast to the two ends of the chain, lest we lose the 

 intermediate links." This argument of the embarrassed theologian 

 would be ridiculous if we were to attribute to physical theories the 

 interpretation given them by the man of the world. In case of con- 

 tradiction one of them at least should be considered false. But this is 

 no longer the case if we only seek in them what should be sought. It 

 is quite possible that they both express true relations, and that the 

 contradictions only exist in the images we have formed to ourselves of 

 reality. To those who feel that we are going too far in our limitations 

 of the domain accessible to the scientist, I reply: These questions 

 which we forbid you to investigate, and which you so regret, are not 

 only insoluble, they are illusory and devoid of meaning. 



Such a philosopher claims that all physics can be explained by the 

 mutual impact of atoms. If he simply means that the same relations 

 obtain between physical phenomena as between the mutual im- 

 pact of a large number of billiard balls well and good! this is 

 verifiable, and perhaps is true. But he means something more, and 

 we think we understand him, because we think we know what an 

 impact is. Why? Simply because we have often watched a game of 

 billiards. Are we to understand that God experiences the same sensa- 

 tions in the contemplation of His work that we do in watching a 

 game of billiards? If it is not our intention to give his assertion 

 this fantastic meaning, and if we do not wish to give it the more re- 

 stricted meaning I have already mentioned, which is the sound mean- 

 ing, then it has no meaning at all. Hypotheses of this kind have there- 

 fore only a metaphorical sense. The scientist should no more banish 

 them than a poet banishes metaphor ; but he ought to know what they 

 are worth. They may be useful to give satisfaction to the mind, and 

 they will do no harm as long as they are only indifferent hypotheses. 



These considerations explain to us why certain theories, that were 



