In that form of desire which is called "attention," the train of thought, held fast, for a time, in the desired direction, seems ever striving to get on to another line--and the junctions and sidings are so multitudinous! The constituents of trains of ideas may be grouped in various ways.
Hume says:-- "We find, by experience, that when any impression has been present in the mind, it again makes its appearance there as an idea, and this it may do in two different ways: either when, on its new appearance, it retains a considerable degree of its first vivacity, and is somewhat intermediate between an impression and an idea; or when it entirely loses that vivacity, and is a perfect idea.

The faculty by which we repeat our impressions in the first manner, is called the _memory_, and the other the _imagination_."-- (I.pp.

23, 24.) And he considers that the only difference between ideas of imagination and those of memory, except the superior vivacity of the latter, lies in the fact that those of memory preserve the original order of the impressions from which they are derived, while the imagination "is free to transpose and change its ideas." The latter statement of the difference between memory and imagination is less open to cavil than the former, though by no means unassailable.
The special characteristic of a memory surely is not its vividness; but that it is a complex idea, in which the idea of that which is remembered is related by co-existence with other ideas, and by antecedence with present impressions.
If I say I remember 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.