THE EXPLANATION OF LAWS 83 



propositions (i) that the pressure of a gas increases as the 

 temperature increases (2) that it increases as the volume 

 decreases, can be deduced from the single proposition 

 that the pressure increases with increase of temperature 

 and decrease of volume. But of course the single pro- 

 position does not explain the two others ; it merely states 

 them in other words. But that is just what logical deduc- 

 tion consists of ; to deduce a conclusion from premisses 

 is simply to state the premisses in different words, though 

 the words are sometimes so different as to give quite a 

 different impression. l If all that we required of a theory 

 was that laws could be deduced from it, there would be 

 no difference between a theory which merely expressed 

 the laws in different words without adding anything 

 significant and a theory which, like the example we are 

 considering, does undoubtedly add something signifi- 

 cant. 



It is clear then that when we say the theory explains 

 the laws we mean something additional to this mere 

 logical deduction ; the deduction is necessary to the 

 truth of the theory, but it is not sufficient. What else do 

 we require ? I think the best answer we can give is that, 

 injorder that j^Jj^gp^ mav exglajs, we re< l iure it to 

 explainl We7equirel:liiar^ to ourjdgaSj and 



that the idea wTucir'it adds' sEall be acceptable. The 

 reader will probably feel that this is true of the explana- 

 tion of the properties of gases offered by the dynamical 

 theory. Even if he did not know (and he probably does 

 not know apart from what I have just told him) that the 

 laws can be deduced from the theory, he would feel that 

 the mere introduction of moving particles and the sugges- 

 tion that the properties of a gas can be represented as 

 due to their motion would afford some explanation of 

 those properties. They would afford some explanation, 



1 The reader should be warned that some people would dissent 

 vehemently from this assertion, 



