The purpose of this paper is to develop the thought that delusional formations fulfil a definite function; they are the expression of certain underlying trends in the individual, and they satisfy certain needs. At the present time there is a reaction against the tendency to regard classification as the ultimate aim of clinical psychiatry. While the separation of mental disorders into certain broad groups has its obvious uses, it is being recognised that classification in itself is not a very vital point, and it does not take us far in the understanding of our cases. Not only is each case a member of a particular group, but in a certain sense it is an entity in itself; the odd behaviour, the delusions and hallucinations, have an individual significance; they are the outgrowth of personal conflicts and aspirations, and the whole psychosis is no more than one form of reaction to experience.