Any disquisition upon dreams which seeks to explain as many as possible of their observed characteristics from a particular point of view, and which at the same time defines the position occupied by dreams in a wider sphere of phenomena, deserves to be called a theory of dreams. The various theories will be found to differ in that they select one or the other characteristic of dreams as the essential one and take it as the point of departure for their explanations and correlations. It need not necessarily be possible to infer a. function of dreaming (whether utilitarian or otherwise) from the theory. Nevertheless, since we have a habit of looking for teleological explanations, we shall be more ready to accept theories which are bound up with the attribution of a function of dreaming.

Sigmund Freud, The Interpretation of Dreams