168 WHAT IS SCIENCE? 



in what the desired product consists, and exactly in 

 what particulars it differs from the unsatisfactory product. 

 This problem of determining precisely what are the exist- 

 ing facts is not strictly one of science at all ; the solution 

 of it does not involve the statement of any scientific laws, 

 for laws assert, not what does actually occur, but what 

 will occur if something else occurs. Nevertheless science 

 and scientific laws are useful, and even indispensable, in 

 the solving of it ; for very often the best or only proof 

 that something has occurred is that some other event 

 has occurred with which the first is associated by a law. 

 Thus, the physician bases his diagnosis on his examina- 

 tion of symptoms ; he observes that the bodily state of 

 his patient is abnormal in some particulars, and from his 

 knowledge of the laws connecting those parts of the body 

 which are accessible to observation with those that are 

 not deduces what must be the state of the hidden organs. 

 In the same way the works chemist or physicist is often 

 led to judge what is the source of failure in a product, 

 by examining carefully the process by which it has been 

 produced, and deducing by his knowledge of the laws 

 of chemistry or physics what must be the result of that 

 process. 



It is for such reasons that pure science, although it 

 takes no direct cognizance of the actual events of practical 

 life, is of inestimable service in explaining and controlling 

 them. Even though it is impossible to analyse those 

 events completely into laws, it is only by carrying that 

 analysis as far as it will go, and by bringing to light all 

 the laws that are involved, that any explanation or 

 any control can be attained. 



These considerations have been suggested by the first 

 of the two errors noted on page 164 ; they answer the 

 objection that science is not possessed of any positive 

 and certain knowledge, or that, if it has such knowledge, it 

 is not relevant to practical problems. The second 



