member A. B., the chance acquaintance of ten years ago, it
is not because my idea of A. B. is very vivid--on the contrary, it is
extremely faint--but because that idea is associated with ideas of
impressions co-existent with those which I call A. B.; and that all
these are at the end of the long series of ideas, which represent that
much past time. In truth I have a much more vivid idea of Mr. Pickwick,
or of Colonel Newcome, than I have of A. B.; but, associated with the
ideas of these persons, I have no idea of their having ever been derived
from the world of impressions; and so they are relegated to the world of
imagination. On the other hand, the characteristic of an imagination may
properly be said to lie not in its intensity, but in the fact that, as
Hume puts it, "the arrangement," or the relations, of the ideas are
different from those in which the impressions, whence these ideas are
derived, occurred; or in other words, that the thing imagined has not
happened. 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