NOMENCLATURE OF MENTAL OPERATIONS m 



same time, taken notice of a third group, of no 

 small importance, which are as different from 

 imaginations as memories are; though, like the 

 latter, they are often confounded with pure 

 imaginations in general speech. These are the 

 ideas of expectation, or as they may be called 

 for the sake of brevity, Expectations; which 

 differ from simple imaginations in being associated 

 with the idea of the existence of corresponding 

 impressions, in the future, just as memories con- 

 tain the idea of the existence of the corresponding 

 impressions in the past. 



The ideas belonging to two of the three groups 

 enumerated: namely, memories and expectations, 

 present some features of particular interest. And 

 first, with respect to memories. 



In Hume's words, all simple ideas are copies of 

 simple impressions. The idea of a single sensa- 

 tion is a faint, but accurate, image of that sensa- 

 tion; the idea of a relation is a reproduction of 

 the feeling of co-existence, of succession, or of 

 similarity. But, when complex impressions or 

 complex ideas are reproduced as memories, it is 

 probable that the copies never give all the details 

 of the originals with perfect accuracy, and it is 

 certain that they rarely do so. No one possesses 

 a memory so good, that if he has only once 

 observed a natural object, a second inspection does 

 not show him something that he has forgotten. 

 Almost all, if not all, our memories are therefore 



