Sensitives through whom physical phenomena (including materializations) can be produced have been described, firstly, as persons in whom certain forces are stored up, either far in excess of the amount possessed by the normal man or woman, or else differing in quality from the forces stored up by the normal man or woman; and secondly, as persons who are able to attract from those in close proximity to them--provided that the conditions are favorable--still more of the force, which thus becomes centered in them for the time being. In other words, a sensitive for physical phenomena is said to be a storage battery for the force which is used in the production of physical phenomena--including materializations--although it is by no means improbable that such highly developed sensitives as those required for this special purpose may be found to possess extra nerve-centers as compared with those possessed by normal human beings. But whether this hypothesis be eventually proved or not, there seems to be but very little doubt that "whatever the force may be which constitutes the difference between a sensitive and a non-sensitive, it is certainly of a mental or magnetic character, i.e., a combination of the subtle elements of mind and magnetism, and therefore of a _psychological_, and not of a purely _physical_ character." But why is a sensitive necessary? you ask. Think of a telephone for a moment. You wish to communicate with a person who is holding only the end of the wire in his hand, the result being that he cannot hear a single word. Why is this? Because he has forgotten to fit a receiver at his end of the wire, a receiver in which the vibrations set up by your voice may be centralized, focussed, a receiver which he can place to his ear, and in doing so will at once hear your voice distinctly--but without this your message to him is lost. And it is said that this is exactly the use of the sensitives during our experiments, for they act as "receivers" in which the forces employed in the production of the phenomena may be centralized, focussed, their varying degrees of sensitiveness enabling them to be used by the entities in other spheres for the successful production of such phenomena, we are told. And lastly, we require about twelve to sixteen earnest and really sympathetic men and women--persons trained on scientific lines for choice--all in the best of health; men and women who, whilst strictly on their guard against anything in the shape of fraud, are still so much in sympathy with the person who is acting as the sensitive that they are all the time sending out kindly thoughts towards him; for if, as has been said, "thoughts are things," it is possible that hostile thoughts would be sufficient not only to enfeeble, but actually to check demonstrations of physical phenomena of all kinds in the presence of such specially organized, highly developed individuals as the sensitives through whom materializations can be produced.