224 ON THE RELATION OF THE 



for tlie fact that we must have it about us in this form, black upon 

 white, proves that we have not mastered it intellectually. It is not suf- 

 ficient to be cognizant of facts ; science results only fi?om u knowledge 

 of their laws and causes. The logical elaboration of these facts consists 

 in collecting together those which are similar under one common head. 

 Thus are formed generic ideas, which take their place in our thinking. 

 We call them generic ideas when they comprise a number of existing 

 things, and laws when they comprise a series of i)henomena or processes. 

 When I have discovered that all the mammalia, i. {?., all warm-blooded 

 animals which bring forth living young, breathe by means of lungs, 

 have two chambers of the heart and at least three auricular bones, I need 

 no longer remember these peculiarities separately for the ape, the horse, 

 the dog, or the whale. The general rule includes an immense number of 

 individual instances and represents them in the memory. The law of the 

 refraction of light does not only include all cases where rays fall, at 

 ditferent angles, upon a smooth surface of water and show the result, 

 but all cases where rays of any color strike a surface of any kind of any 

 transparent substance. This law, therefore, includes such an endless 

 number of cases that it would have been absolutely impossible to retain 

 them all singly in the memory. Moreover, this law does not only in- 

 clude those cases which we or others have already observed, but we do 

 not hesitate to apply it to new cases, which have not yet been recog- 

 nized, to predict the effect of the refraction of light, and our expecta- 

 tions will not be disappointed. In the same manner, if we should find 

 an unknown mammal, that has never been anatomically dissected, we 

 might conclude almost with certainty that it had lungs, two heart- 

 chambers, and three or more auricular bones. 



While we thus generalize the facts of our experience into classes and 

 laws, we not only reduce our knowledge to a form in which it is more 

 easily used and remembered, but we actually increase it, since we can 

 extend the rules and laws thus found to cases which may come to our 

 notice in future. 



In the above examples the generalization of facts presents no diffi- 

 culty, and the whole process is obvious. But in complicated cases we 

 do not succeed so easily in separating the similar from the dissimilar, and 

 in forming clear, sharply defined ideas. Suppose we know a man to be 

 ambitious ; we may predict, with tolerable certainty, that, if this man 

 be placed in certain conditions, he will follow the promptings of his 

 ambition and choose a certain course of action. But we can neither 

 define with certainty how an ambitious man is to be recognized, nor 

 how his ambition can be estimated, nor can we ascertain how great it 

 must be to lead him, under certain circumstances, to adopt a certain 

 line of action. We compare the observed actions of one man with those 

 of other men who have acted similarly in similar cases, and draw our 

 conclusion as to the result of future actions, without having either our 

 major or oiu' minor premise clearly defined, and even without being 



