The origin of manic-depressive illness is unknown. We have only working hypotheses, none of which has so far thrown much light on the mystery. Heredity is no doubt important, and so are psychological factors. The thesis of this book is that 'the basic manic-depressive illness always has its origin in physiologic susceptibility which is subject to the releasing, modifying and prolonging influence of psychic factors.' This thesis is not new but still in need of confirmation. The author believes that dysfunction of the diencephalic-rhinencephalic-reticular systems, which have recently been much in the news, is an important etiological factor. At times he gives the impression that this is proved already. Endocrine factors are also regarded as significant in the causation of the illness. The presentation is discursive and often unnecessarily dogmatic. The uninitiated reader will find it difficult to distinguish between the established and the hypothetical. The book is not likely to impress those who lean towards the author's views, or those who do not share them.