2 THE NATURAL HISTORY OF THE COMMON CRAYFISH. 



for the commonest purposes of everyday existence. 

 Common sense is science exactly in so far as it fulfils 

 the ideal of common sense ; that is, sees facts as they 

 are, or, at any rate, without the distortion of prejudice, 

 and reasons from them in accordance with the dictates 

 of soimd judgment. And science is simply common sense 

 at its best ; that is, rigidly accurate in observation, and 

 merciless to fallacy in logic. 



Whoso will question the validity of the conclusions of 

 sound science, must be prepared to carry his scepticism 

 a long way ; for it may be safely af&rmed, that there is 

 hardly any of those decisions of common sense on 

 which men stake their all in practical life, which can 

 justify itself so thoroughly on common sense principles, 

 as the broad truths of science can be justified. 



The conclusion drawn from due consideration of the 

 nature of the case is verified by historical inquiry ; and 

 the historian of every science traces back its roots to the 

 primary stock of common information possessed by all 

 mankind. 



In its earliest development knowledge is self-sown. 

 Impressions force themselves upon men's senses whether 

 they will or not, and often against their will. The 

 amount of interest which these impressions awaken is 

 determined by the coarser pains and pleasures which 

 they carry in their train, or by mere curiosity; and 

 reason deals with the materials supplied to it as far as 

 that interest carries it, and no farther. Such common 



