THE EXPLANATION OF LAWS 85 



developing our view further, it will be well to examine 

 the matter from another point of view. 



DIFFERENCE BETWEEN THEORIES AND LAWS 



It was stated before that it has been usually held that 

 the explanation of laws consists in showing that they are 

 particular examples of more general laws. If this view 

 were applied to the example under discussion, it might 

 be urged that the dynamical theory explains the properties 

 of gases because it shows that they are particular 

 examples of the laws of dynamics ; the properties of 

 gases are explained because they are shown to be the 

 consequences of the subjection of the molecules, of which 

 the gases consist, to the general laws of all moving bodies. 

 Here, it might be said, is the clearest possible instance of 

 explanation by generalization, a simple extension of the 

 process involved in the discovery of laws. 



But, against this view, it must be pointed out that the 

 most important feature of the theory is not that it states 

 that molecules are subject to dynamical laws, but that : 

 which states that there are such things as molecules, and j 

 that gases are made up of them. It is that feature of the / 

 theory which makes it a real explanation. Now this 

 part of the theory is not a particular instance of any more 

 general law ; indeed it is not a law or anything that could 

 be an instance of a law. For it is not, according to the 

 criterion laid down in Chapter II, part of the proper 

 subject-matter on which science builds its foundations. 

 Molecules are not things which we can see or feel ; they 

 are not, like the ordinary material bodies to which the 

 laws of dynamics are known to apply, objects discernible 

 to direct perception. We only know that they exist 

 by inference ; what we actually observe are gases, vary- 

 ing in temperature and pressure ; and it is only by these 

 variations that we are led to suspect the existence of the 



