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.