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Clairvoyance means literally nothing more than "clear seeing," and it is a word which has been sorely misused, and even degraded so far as to be employed to describe the trickery of a mountebank in a variety show. For the purpose of this treatise we may, perhaps, define it as the power to see what is hidden from ordinary physical sight. Let me make two points clear before I begin. First, I am not writing for those who do not believe that there is such a thing as clairvoyance, nor am I seeking to convince those who are in doubt about the matter. Before a detailed explanation of clairvoyance can usefully be attempted, however, it will be necessary for us to devote a little time to some preliminary considerations, in order that we may have clearly in mind a few broad facts as to the different planes on which clairvoyant vision may be exercised, and the conditions which renders its exercise possible.