14 ON THE RELATION OF 



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 sur- 

 face 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 whatsoever. 

 This law, therefore, includes an infinite number of cases, 

 which it would have been absolutely impossible to carry 

 in one's memory. Moreover, it should be noticed that 

 not only does this law include the cases which we our- 

 selves or other men have already observed, but that we 

 shall not hesitate to apply it to new cases, not yet ob- 

 served, 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 

 process of thought, and form conceptions, whether general 

 conceptions or laws, we not only bring our knowledge 

 into a form in which it can be easily used and easily re- 

 tained, 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 diffi- 

 culties, 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 



