GENERALIZA TION. 263 



may present to the senses very different appearances. 

 Without a careful analysis of the changes which take 

 place, we may often be in danger of widely separating 

 facts and processes, which are actually instances of the 

 same law. Extreme difference of degree or magnitude 

 is a very frequent cause of error. It is truly difficult 

 at the first moment to recognise any similarity between 

 the gradual rusting of a piece of iron, and the rapid 

 combustion of a heap of straw. Yet Lavoisier's chemical 

 theory was founded upon the close similarity of the oxy- 

 dizing process in one case and the other. We have only 

 indeed to divide the iron into excessively small particles 

 to discover that it is really the more combustible of the 

 two, so that it actually takes fire spontaneously and burns 

 like tinder. It is the excessive slowness of the process in 

 the case of a massive piece of iron which disguises its real 

 character. 



If Xenophon reports truly, Socrates was seriously mis- 

 led by not making sufficient allowance for extreme differ- 

 ences of degree and quantity. He rejected the acute 

 opinion of Anaxagoras that the sun is a fire, on the ground 

 that we can look at a fire, but not at the sun, and that 

 plants grow by sunshine while they are killed by fire. 

 He also pointed out that a stone heated in a fire is not 

 luminous, and soon cools, whereas the sun ever remains 

 equally luminous and hot '. All such mistakes evidently 

 arise from not perceiving that difference of quantity may 

 be so extreme as to assume the appearance of difference 

 of quality. It is the least creditable thing we know of 

 Socrates, that when pointing out these supposed mistakes 

 of earlier philosophers, he advised his followers not to 

 study astronomy. 



Masses of matter of very different size may always be 



q ' Memorabilia,' iv. 7 ; quoted by "Whewell, ' History of Inductive 

 Sciences,' vol. i. p. 340. 



