It has long been a favourite tenet of mine, and there are now, I think, others who hold it, that Insanity is no exception to the rule which requires a knowledge of the normal as an indispensable preliminary to a knowledge of the abnormal. The reason why the contrary opinion has been maintained with such vigour, and the contrary practice so generally followed, has seemed to me to be the absence of any work in which normal psychological processes are dealt with from the point of view and for the purposes of the alienist. Of the many excellent works on Psychology which are at the service of the student, there is none that affords him material help in understanding the nature of those disorders of mind which it is the work of his life to study. For instance, the chief labours of the student of the disordered mind are concerned with the existence and nature of Delusion; but, as far as I know, no work on normal psychology gives him any help in settling the preliminary questions of what a delusion is; of how it differs from a normal state of mind; of its mode of origin; or of its varieties. It is true that these are not questions in normal psychology, and it is no reproach to the psychologist who deals with the normal alone that they are excluded from his purview; but it is a great disadvantage to the alienist to be left without guidance in the face of problems of such profound importance to him.