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From the economic standpoint psycho-analysis supposes that the mental representatives of the instincts have a charge (cathexis) of definite quantities of energy, and that it is the purpose of the mental apparatus to hinder any damming-up of these energies and to keep as low as possible the total amount of the excitations with which it is loaded. The course of mental processes is automatically regulated by the ‘pleasure-unpleasure principle’; and unpleasure is thus in some way related to an increase of excitation and pleasure to a decrease. In the course of development the original pleasure principle undergoes a modification with reference to the external world, giving place to the ‘reality principle’, in accordance with which the mental apparatus learns to postpone the pleasure of satisfaction and to tolerate temporarily feelings of unpleasure.

Topographically, psycho-analysis regards the mental apparatus as a compound instrument, and endeavours to determine at what points in it the various mental processes take place. According to the most recent psycho-analytic views, the mental apparatus is composed of an ‘id’, which is the repository of the instinctual impulses, of an ‘ego’, which is the most superficial portion of the id and one which has been modified by the influence of the external world, and of a ‘super-ego’, which develops out of the id, dominates the ego and represents the inhibitions of instinct that are characteristic of man. The quality of consciousness, too, has a topographical reference; for processes in the id are entirely unconscious, while consciousness is the function of the ego’s outermost layer, which is concerned with the perception of the external world.

At this point two observations may be in place. It must not be supposed that these very general ideas are presuppositions upon which the work of psycho-analysis depends. On the contrary, they are its latest conclusions and are ‘open to revision’. Psycho-analysis is founded securely upon the observation of the facts of mental life; and for that very reason its theoretical superstructure is still incomplete and subject to constant alteration. Secondly, there is no reason for surprise that psycho-analysis, which was originally no more than an attempt at explaining pathological mental phenomena, should have developed into a psychology of normal mental life. The justification for this arose with the discovery that the dreams and mistakes of normal men have the same mechanism as neurotic symptoms.