IK EXPLANATION or 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 

 \vords are sometimes so different as to give quite a 

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

 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 

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

 rder that a theory may explain, we require it to 

 We require that it shall add to our ideas, and 

 ii it adds shall be acceptable. The 

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

 tion of the properties of gases offered by tin dynamical 

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

 know apart from what I hav< Id him) that the 



can be deduced from the theory, he would fed that 

 reduction of moving particles and the sugges- 

 t the properties of a gas can be represented as 

 D would afford some explanation of 

 those proper would afford some explanation, 



reader should be warned tliat some people would dissent 

 ly from this assert i 



