148 HUME vi 



the one, by which it infallibly produces the other, and 

 operates with the greatest certainty and strongest neces- 

 sity. . . . 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 existence ; 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 im- 

 provement was made down to the time of the pub- 

 lication of Mill's " Logic/' Of Mill's four meth- 

 ods, 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 dis- 



