THE APPLICATIONS OF SCIENCE 167 



is not only a consequence of the laws, it is a consequence 

 of the laws and of the other events to which it is related 

 by the laws. Again, it must be noted that I have spoken 

 of laws, not " a law/' The practical event to which our 

 attention is directed will not be a simple event such as 

 is related by pure science to another simple event ; it 

 will be an immensely complicated collection of such 

 events, and these constituent events will be each related 

 to some other event by a separate law. The consti- 

 tuent events will not in general be related to each other 

 by a law, nor will the other events, to which they are so 

 related, be related to each other by a law. The explana- 

 tion of the events in question will not be complete when 

 we have stated a single law, or even all the many laws 

 in which the constituent events are involved ; it is neces- 

 sary to add that the many events to which they are related 

 by these many laws have actually occurred, and that the 

 many laws are actually in operation. 



The last part of the explanation is the part which is 

 not pure science. Science when it asserts laws, only 

 asserts that, if so-and-so happens, something else will 

 also happen. But in practical matters it is necessary to 

 convert this hypothetical statement into a definite state- 

 ment, and to assert that something actually has happened. 

 This is often an extremely difficult matter, which may be 

 the subject of much difference of opinion _ until all the 

 circumstances have been investigated. An obvious 

 example of this difficulty arises in the practice of 

 medicine ; diagnosis, the determination of what is wrong 

 with the patient, is a necessary preliminary to his treat- 

 ment, and is actually the gravest problem with which 

 the physician is faced. And similar problems arise in all 

 other branches of applied science. If we are asked to 

 produce some desired product or to find out why the 

 product of some existing process is not satisfactory, the 

 first part of our task must always be to find out exactly 



