But, for reasons already given, no case should be suffered to rest upon a single memory. It is of the highest importance, therefore, to obtain the corroborative testimony of persons who were cognisant of the occurrence of the impression before the news of the corresponding event. When this is not to be obtained, evidence of some unusual action on the part of the percipient, such as the taking of a journey, or the putting on of mourning, may be accepted as collateral proof of the reality of his impression. But, as we have already seen, the evidence of the attesting witnesses is liable to the same errors which affect the testimony of the percipient; and the evidence most to be desired is of a kind exempt from these weaknesses--that of a letter or memorandum written before the news. In a large proportion of the narratives dealt with, it is asserted that such a letter was written, or such a memorandum made. Unfortunately, this alleged documentary evidence is rarely forthcoming. It is possible that in some cases this statement is merely a conventional dramatic tag,--an addition made unconsciously and in perfect good faith to round off the story.[82] It cannot, however, I think, be regarded as surprising either that a letter or note was not written at the time, or that, if written, it should not have been preserved. Sensory hallucinations--to take the most striking instance--though unusual are not extremely rare experiences; most educated persons are perfectly familiar with the fact of their occurrence and regard them (in most cases rightly) as purely subjective, the products of some transient cerebral disturbance, as little worthy of record as a headache or a bilious attack. Often, probably, the telepathic hallucination is indistinguishable from the mass of purely subjective experiences of the same kind; and even should it be recognised at the time as exceptional, the want of leisure, the fear of ridicule, even the dislike of seeming to admit to himself the possibility of his experience having a sinister significance, would probably deter the percipient from writing about it.[83] It is much more likely that he would speak of it to an intimate friend, should opportunity occur. And when in the rare conjunction of an exceptional experience, adequate leisure, and a sympathetic correspondent, or the habit of writing a diary, the letter is actually written or the note made, the chances which militate against its preservation are many. Few persons will take a general and impersonal (in other words, a scientific) interest in occurrences of this kind. Their own isolated experience may possess a deep and abiding interest for themselves, and, less certainly, for their friends; an interest, however, which is quite compatible with the treatment of the attesting record as waste paper. But unless it can be used to illustrate or support a theory of a future life, they seldom regard a "ghost story" as having any value