THE SCIENTIFIC MOOD. 17 



the lecturer says that a gas " obeys Boyle's Law," he 

 is using the language of the past and suggesting a 

 conception of the order of nature which is no longer 

 current. " We must confess," says Prof. J. J". 

 Poynting,* " that physical laws have greatly fallen 

 off in dignity. No long time ago they were quite 

 commonly described as the Fixed Laws of Nature, 

 and were supposed sufficient in themselves to govern 

 the universe. Now we can only assign to them the 

 humble rank of mere descriptions, often tentative, 

 often erroneous, of similarities which we believe we 

 have observed." 



Prof. Poynting goes on to say that a " law of na- 

 ture explains nothing it has no governing power, 

 it is but a descriptive formula which the careless 

 have sometimes personified. There may be psycho- 

 logical and social generalisations which really tell 

 us why this or that occurs, but chemical and phys- 

 ical generalisations are wholly concerned with the 

 how." 



In other words, if we may condense a little of 

 Poynting's admirable discourse, concurrently with 

 the change in our conception of physical law has 

 come a change in our conception of physical expla- 

 nation. The change is in our recognition that " we 

 explain an event not when we know ' why ' it hap- 

 pened, but when we know ' how ' it is like something 

 else happening elsewhere or otherwise when, in 

 fact, we can conclude it as a case described by some 

 law already set forth. In explanation we do not 

 account for the event, but we improve our account of 

 it by likening it to what we already knew." In 

 short, the notion of antecedent purpose which rises 



* Address, Section A, Report of British Ass. for 1889, pp. 

 616-17. 



