This idea of putting the problem of knowledge in terms exactly the reverse of those in which it seems “natural” to put it was originally suggested to Bergson by his study of the important work on amnesia carried out by Charcot and his pupils, and also by such evidence as was to be had at the time when he wrote on the curious memory phenomena revealed by the use of hypnotism and by cases of spontaneous dissociation. It is impossible to prove experimentally that no experience is ever destroyed but it is becoming more and more firmly established that enormous numbers of past experiences, which are inaccessible to ordinary memory and which therefore it would seem “natural” to suppose destroyed, can, if the right methods are employed, be revived even with amazing fullness of detail. In recent years since Bergson’s books were first published, great strides have been made in the experimental investigation of the whole subject of memory, and the evidence thus obtained, far from upsetting the theory of memory suggested to him by the less extensive evidence which was available at the time when he wrote, lends it striking support. It appears to be accepted by doctors who use hypnotism in psychotherapy that under hypnotism many patients can perfectly well be taken back in memory to any period of their lives which the doctor chooses to ask for, and can be made not only to remember vaguely a few incidents which occurred at the time but actually to re-live the whole period in the fullest possible detail, feeling over again with hallucinatory vividness all the emotions experienced at the time. This re-living of past experience can, with some patients, be made to go on indefinitely, through the whole day, if the doctor has time to attend to it, every little incident being faithfully recalled though the actual event may have taken place 20 or 30 years previously. And this happens not simply in the case of some very striking event or great crisis which the patient has been through, indeed it is just the striking events that are often hardest to recover. Some doctors, in order to get at the crisis, have found it useful occasionally to put patients back through one birthday after another right back even as early as their second year, to see at what point in their lives some particular nervous symptom first appeared, and each successive birthday is lived through again in the utmost detail.[4] [4] See _Psychology and Psychotherapy_ by Dr. William Brown.