hirls about the conception of the object of regret or remorse
as a centre; every now and then, indeed, breaking away into the new
tracks suggested by passing associations, but still returning to the
central thought. Few can have been so happy as to have escaped the
social bore, whose pet notion is certain to crop up whatever topic is
started; while the fixed idea of the monomaniac is but the extreme form
of the same phenomenon.

And as, on the one hand, it is so hard to drive away the thought we
would fain be rid of; so, upon the other, the pleasant imaginations
which we would so gladly retain are, sooner or later, jostled away by
the crowd of claimants for birth into the world of consciousness; which
hover as a sort of psychical possibilities, or inverse ghosts, the
bodily presentments of spiritual phenomena to be, in the limbo of the
brain. 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 re