12 ON THE RELATION OK 



made out that all mammals that is, all warm-blooded, vivi- 

 parous animals breathe through lungs, have two chambers in 

 the heart, and at least three tympanal bones, I need no longer 

 remember these anatomical peculiarities in the individual cases 

 of the monkey, the dog, the horse, and the whale; the general 

 rule includes a vast number of single instances, and represents 

 them in my memory. When I enunciate the law of refraction, 

 not only does this law embrace all cases of rays falling at all 

 possible angles on a plane surface of water, and inform me of 

 the result, but it includes all cases of rays of any colour incident 

 on transparent surfaces of any form and any constitution what- 

 soever. This law, therefore, includes an infinite number of 

 cases, which it would have been absolutely impossible to carry 

 in one's memoiy. Moreover, it should be noticed that not only 

 does this law include the cases which we ourselves or other men 

 have already observed, but that we shall not hesitate to apply it 

 to new cases, not yet observed, with absolute confidence in the 

 reliability of our results. In the same way, if we were to find 

 a new species of mammal, not yet dissected, we are entitled to 

 assume, with a confidence bordering on a certainty, that it has 

 lungs, two chambers in the heart, and three or more tympanal 

 bones. 



Thus, when we combine the results of experience by a pro- 

 cess of thought, and form conceptions, whether general concep- 

 tions or laws, we not only bring our knowledge into a form in 

 which it can be easily used and easily retained, but we actually 

 enlarge it, inasmuch as we feel ourselves entitled to extend the 

 rules and the laws we have discovered to all similar cases that 

 may be hereafter presented to us. 



The above-mentioned examples are of a class in which the 

 mental process of combining a number of single cases so as to 

 form conceptions is unattended by farther difficulties, and can be 

 distinctly followed in all its stages. But in complicated cases it 

 is not so easy completely to separate like facts from unlike, and 

 to combine them into a clear well-defined conception. Assume 

 that we know a man to be ambitious ; we shall perhaps be able 

 to predict with tolerable certainty that if he has to act under 



