We will now go on to the third class of mnemonic error, which I have called the spectra of memory, where there is not simply a transformation of the past event, but a complete imaginative creation of it.  This class of error corresponds, as I have observed, to an hallucination in the region 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 false.  Such a person, if he purchases freely, is pretty sure to make a number of mistakes.  Similarly, all of us are liable to take counterfeit mnemonic images for genuine ones; that is to say, to fall into an illusion of “recollecting” what never really took place.