VI CONCERNING NECESSARY TRUTHS 147 



greatest certainty and strongest necessity. . . . But there is 

 nothing in a number of instances, different from every single 

 instance, which is supposed to be exactly similar ; except only, 

 that after a repetition of similar instances, the mind is carried by 

 habit, upon the appearance of one event, to expect its usual 

 attendant, and to believe that it will exist. . . . The first time 

 a man saw the communication of motion by impulse, as by the 

 shock of two billiard balls, he could not pronounce that the one 

 event was connected, but only that it was conjoined, with the 

 other. After he has observed several instances of this nature, 

 he then pronounces them to be connected. What alteration has 

 happened to give rise to this new idea of connexion ? Nothing 

 but that he now feels these events to be connected in his 

 imagination, and can readily foresee the existence of the one 

 from the appearance of the other. When we say, therefore, that 

 one object is connected with another we mean only that they 

 have acquired a connexion in our thought, and give rise to this 

 inference, by which they become proofs of each other's exist- 

 ence ; a conclusion which is somewhat extraordinary, but which 

 seems founded on sufficient evidence." (IV. pp. 87 89.) 



In the fifteenth section of the third part of the 

 " Treatise," under the head of the Rules by which to 

 Judge of Causes and Effects, Hume gives a sketch 

 of the method of allocating effects to their causes, 

 upon which, so far as I am aware, no improvement 

 was made down to the time of the publication of 

 Mill's " Logic." Of Mill's four methods, that of 

 agreement is indicated in the following passage : 



"... where several different objects produce the same effect, 

 it must be by means of some quality which we discover to be 

 common amongst them. For as like effects imply like causes, 

 we must always ascribe the causation to the circumstance 

 wherein we discover the resemblance." (I. p. 229.) 



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