Further thought, however, raises difficulties and doubts. The gradually recognized possibility of what may be called normal telepathy, or unconscious mind-reading from survivors, raises hesitation--felt most by studious and thoughtful people--about accepting such messages as irrefragable evidence of persistent personal existence; and to overcome this curious and unexpected and perhaps rather artificial difficulty, it is demanded that facts shall be given which are unknown to anyone present, and can only subsequently be verified. Communications of this occasional and exceptional kind are what are called, by psychic investigators, more specifically 'evidential': and time and perhaps good fortune may be required for their adequate reception and critical appreciation. For it is manifest that most things readily talked about between two friends, and easily reproducible in hasty conversation, will naturally be of a nature common to both, and on subjects well within each other's knowledge. The more recent development of an elaborate scheme of 'cross-correspondence,' entered upon since the death of specially experienced and critical investigators of the S.P.R., who were familiar with all these difficulties, and who have taken strong and most ingenious means to overcome them, has made the proof, already very strong, now almost crucial. The only alternative, in the best cases, is to imagine a sort of supernormal mischievousness, so elaborately misleading that it would have to be stigmatised as vicious or even diabolical.