THE LAWS OF SCIENCE ,53 



of a substance, the answer is undoubtedly that it has 

 not been recognized sufficiently that such propositions 

 are laws. For they are not usually called laws. But 

 the fact that the name is not applied to them is largely 

 the result of history. As we noted, laws of this type 

 are among the results which science accepts in the 

 first instance from the experience of common sense, 

 although it subsequently refines them and may change 

 them almost beyond recognition. Knowledge is dignified 

 by the imposing name of law only when it has been 

 arrived at by deliberate and conscious investigation, and 

 not when, like Topsy, it simply " growed." But it is 

 more difficult to explain why numerical laws, to which 

 the name " law " is applied characteristically, have not 

 been recognized as providing instances to show that 

 cause and effect is not the only relation with which laws 

 are concerned. 



I think the real reason is to be found in a confusion 



een the method by which knowledge is attained 



and the content of the knowledge once it is attained. 



What I mean is this. Suppose we were seeking to 



discover whether Ohm's Law is true. We shall set up 



instruments for measuring the current and the pressure, 



and shall then watch how the current changes when we 



change the pressure. In making such experiments, what 



actually observe is that a change in current 



follows a change in pressure ; we shall first make 



a change in the pressure and then observe 



a change in the cunvnt ; in other words, during the 



nt the change in current appears as an effect 



of which the change of pressure is the cause. But, 



:<h it may be maintained that it is by observing 



relations of cause and effect t! discover 



the truth of Ohm's Lav :ot these relation- \\hirh 



are stated b-. 



not the relation in time, that is stated by the law. For, 



