of sense-perception. And just as we distinguished between those
hallucinations of sense which arise first of all through some
peripherally caused subjective sensation, and those which want even this
element of reality and depend altogether on the activity of imagination,
so we may mark off two classes of mnemonic hallucination. The false
recollection may correspond to something past--and to this extent be a
recollection--though not to any objective fact, but only to a subjective
representation of such a fact, as, for example, a dream. In this case
the imitation of the mnemonic process may be very definite and complete.
Or the false recollection may be wholly a retrojection of a present
mental image, and so by no stretch of language be deserving of the name
recollection.

It is doubtful whether by any effort of will a person could bring
himself to regard a figment of his present imagination as representative
of a past reality. Definite and complete hallucinations of this sort do
not in normal circumstances arise. It seems necessary for a complete
illusion of memory that there should be something past and recovered at
the moment, though this may not be a real personal experience.[127] On
the other hand, it is possible, as we shall presently see, under certain
circumstances, to create out of present materials, and in a vague and
indefinite shape, pure phantoms of past experience, that is to say,
quasi-mnemonic images to which there correspond no past occurrences
whatever.

All recollection, as we have seen, takes place by means of a present
mental image which returns with a certain degree of vividness, and is
instantaneously identified with some past event. In many cases this
instinctive process of identification proves to be legitimate, for, as a
matter of fact, real impressions are the first and the commonest source
of such lively mnemonic images. But it is not always so. There are other
sources of our mental imagery which compete, so to speak, with the
region of real personal experience. And sometimes these leave behind
them a vivid image having all the appearance of a genuine mnemonic
image. When this is so, it is impossible by a mere introspective glance
to detect the falsity of the message from the past. We are in the same
position as the purchaser in a jet market, where a spurious commodity
has got inextricably mixed up with the genuine, and there is no ready
criterion by which he can distinguish the true from the