Extreme dissociation by reason of a logic-tight compartment is shown in that kind of insanity in which the sufferer behaves as an ordinary being with ordinary actions and ideas except for the influence of a systematised delusion (generally persecutory or grandiose) of most irrational type which is impregnable to explanation or argument. On all other points the man is sane, and the purely mental origin of the disease is suggested by his remaining in good health and without mental deterioration apart from the delusional system, in this respect differing greatly from the sufferers from most other forms of insanity. Some psychiatrists claim to have traced the delusions back to repressions that took place in early life.[6] [Footnote 6: For a fuller account of dissociation I would refer the reader to _The Psychology of Insanity_, by Dr. Bernard Hart, to which I am indebted for the form of some of my ideas. (Cambridge University Press.)] CHAPTER V WATER-DIVINING Water-divining, or dowsing, is accepted in many parts of the world and used as a practical method of locating underground water. Official bodies as well as private individuals employ practitioners of the art, and among people generally there is a strong belief in its genuineness. It is carried out by means of a forked twig, hazel by traditional preference, which is grasped in the dowser's two hands and is said to be twisted upwards by an unknown force when there is water underground. As an addition it is sometimes claimed that the twig will indicate the presence of metals by being twisted downwards. Believers in the twisting of the twig are generally ignorant that it was formerly used in the pursuit and detection of criminals and the finding of buried treasure[7] and that it was being used in the year 1918 to locate a seam of coal. Going farther afield, we learn that the witch-findings practised by African savages are sometimes carried out by means of a stick which points at the victim. [Footnote 7: See Janet, op. cit., p. 368, where he also says: 'Il est probable que, dans quelques campagnes, subsiste encore la croyance aux revelations de la baguette divinatoire.'] Such varied uses demand a new and complicated system of physics if the results are due to any forces external to the diviner, but my own observations satisfy me that we need not overturn our ordinary conceptions of cause and effect to explain the different properties of the divining-rod. When a friend told me of the presence of a dowser in the neighbourhood and gave me a would-be convincing account of how he had seen him at work, how the twig was twisted upwards with such force that my informant was unable to depress it, and how the man was employed by engineers to tell them where to sink wells, I became interested and asked to see the marvel. The resulting experiment, though conducted haphazard, was instructive as regards both water-divining and credulity.