50 INTRODUCTION TO SCIENCE 



change in our conception of physical explanation. 

 The change is in our recognizing that " we explain 

 an event not when we know 'why' it happened, 

 but when we know 'how' it is like something else 

 happening elsewhere or otherwise when, in fact, 

 we can include it as a case described by some law 

 already set forth. In explanation we do not ac- 

 count for the event, but we improve our account 

 of it by likening it to what we already know." 



It is a common problem of science to account 

 for a given state of things, the appearance of 

 an island, a cold summer, a succession of fine 

 sunsets, a shower of gossamer, a butterfly coming 

 out of a cocoon, and so on. In what way does 

 science account for these things? By a descrip- 

 tion of the conditions of their coming about, and 

 in proportion to the completeness and generality 

 of that description is our satisfaction with the 

 account that is given. We are particularly well 

 satisfied when what seemed to be an exception 

 is shown to prove the rule that is to say, when 

 an apparently strange event is shown to con- 

 form to an established law. 



Let us take a concrete case given by Prof. 

 Karl Pearson (Grammar of Science, ed. 1900, p. 

 99). "The law of gravitation is a brief descrip- 

 tion of how every particle of matter in the uni- 

 verse is altering its motion with reference to every 

 other particle. It does not tell us why particles 



