Mesmer's experiment was almost a "fool's experiment," and Mesmer himself was almost a charlatan. Yet Mesmer and his successors,--working from many different points of view, and following many divergent theories,--have opened an ever-widening way, and have brought us now to a position where we can fairly hope, by experiments made no longer at random, to reproduce and systematise most of those phenomena of spontaneous somnambulism which once seemed to lie so tantalisingly beyond our grasp. That promise is great indeed; yet it is well to begin by considering precisely how far it extends. We must not suppose that we shall at once be subduing to our experiment a central, integrated, reasonable Self. We must be content (at first at any rate) if we can affect the personality in the same limited way as hysteria and somnambulism have affected it; but yet can act deliberately and usefully where these have acted hurtfully and at random. It is enough to hope that we may inhibit pain, as it is inhibited for the hysteric; or concentrate attention, as it is concentrated for the somnambulist; or change the tastes and passions, as these are changed in alternating personalities; or (best of all) recover and fix something of that supernormal faculty of which we have caught fugitive glimpses in vision and dream. Our proof of the origination of any phenomenon in the deeper strata of our being must lie in the intrinsic nature of the faculty exhibited;--not in the wisdom of its actual direction. _That_ must often depend on the order given from above the threshold; just as the magic mill of the fable continues magical, although, for lack of the proper formula to stop it, it be still grinding out superfluous salt at the bottom of the sea. This brief introduction will, I hope, show that hypnotism is no disconnected or extraneous insertion into experimental psychology, but rather a summary name for a group of necessary, though empirical and isolated, attempts to bring under control that range of submerged faculty which has already from time to time risen into our observation. The inquiry has been mainly the work of a few distinguished men, who have each of them pushed some useful ideas as far as they could, but whose work has not been adequately supported by successors.