54 WHAT IS SCIENCE? 



if we change our experimental arrangements a little, we 

 shall be able to alter the relation and interchange cause 

 and eff ect ; we shall be able first to alter the current 

 intentionally and then to observe a change in the pressure. 

 But, though we have thus turned cause into effect and 

 effect into cause, we shall regard the experiments as 

 proving the truth of the same law, because the numerical 

 relation will be unaltered ; the same current will still be 

 associated with the same pressure. Thus, as was said 

 at the start, the law states a relation which is not that of 

 cause and effect, although it may be established by 

 observing such a relation ; there is a distinction between 

 the meaning of the law and the evidence on which it is 

 asserted. 



This distinction appears in experiments of all kinds 

 and is hardly separable from the fundamental idea of 

 an experiment. To make an experiment is practically 

 the same thing as to try what is the effect of some cause, 

 and in making it it is impossible not to think of the cause 

 before thinking of the effect. Thus, to revert to an earlier 

 example, if we are proposing to investigate what action 

 damp air has on steel, in order to make the trial we 

 must be thinking about damp air before we can know 

 what that action is. But when we discover that the 

 steel rusts, we see that the rusting is not the effect of the 

 damp air, in the sense that the presence of the damp 

 necessarily precedes the rusting ; we see that the rusting 

 is going on all the time that the steel is exposed to damp ; 

 the exposure and the rusting are concurrent, not con- 

 secutive. 



A confusion between the order in which the processes 

 are present in our minds when we are making experi- 

 ments or observations and the relation which the pro- 

 cesses necessarily bear to each other this confusion is, 

 I believe, the source of the notion, generally prevalent 

 during the last century, that cause and effect (in the 



