To what causes are we to attribute the persistence of this belief in the supernatural? It is useless replying that its persistence is evidence of its truth. That clearly begs the whole question at issue. Mere social heredity will doubtless count for much in this direction. Men do not start their thinking afresh with each generation. It is based upon that of preceding generations; it follows set forms, and is generally influenced by that network of ideas and beliefs into which we are born and from which none of us ever completely escapes. Still that is hardly enough in itself to account for the persistence of supernaturalism. Assuming that originally there existed what was accepted as good evidence for the existence of a supernatural, it is hardly credible that every subsequent generation went on accepting it merely because one generation received evidence of its existence. As organs atrophy for want of exercise, so do beliefs die out in time for want of proof. Some kind of evidence must have been continually forthcoming in order to keep the belief alive and active. It is not a question of whether the evidence was good or bad. All evidence, it is important to bear in mind, is good to some one. The "facts" upon which thousands of people were put to death for witchcraft would not be considered evidence to anyone nowadays, but they were once accepted as good ground for conviction. What kind of evidence is it, then, that has been accepted as proof of the supernatural? Or, to return to Tylor's definition of religion, seeing that the belief in spiritual beings has persisted in every generation, upon what kind of evidence has this belief been nourished? Various replies might be given to this question, all of which may contain some degree of truth, or an aspect of a general truth. In the present enquiry I am concerned with one line of investigation only, one that has been strangely neglected, but which yet, I am convinced, promises fruitful results. In other directions it has been established that a great aid to an understanding of the human organism in times of health is to study its activities under conditions of disease. Abnormal psychology is now a recognised branch of psychology in general, and a glance through almost any recent text-book will show that the two form parts of a natural whole. The normal and the abnormal are in turn used to throw light on each other. And it appears to the present writer that in the matter of religious beliefs a much clearer understanding of their nature, and also of some of the conditions of their perpetuation, may be gained by a study of what has happened, and is happening, in the light of mental pathology.