In popular usage, however, imagination is frequently employed for simple memory--"In imagination I was back in the old times." It is a curious omission on Hume's part that, while thus dwelling on two classes of ideas, _Memories_ and _Imaginations_, he has not, at the 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 contain 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 sensation is a faint, but accurate, image of that sensation; 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 sketches, rather than portraits, of the originals--the salient features are obvious, while the subordinate characters are obscure or unrepresented.
Now, when several complex impressions which are more or less different from one another--let us say that out of ten impressions in each, six are the same in all, and four are different from all the rest--are successively presented to the mind, it is easy to see what must be the nature of the result.