To appreciate the full significance of the aforementioned difference between the two groups of instincts, we must digress considerably and introduce a consideration which we must needs call _economic_. Thereby we enter upon one of the most important but unfortunately one of the most obscure domains of psychoanalysis. We ask ourselves whether a fundamental purpose is recognizable in the workings of our psychological apparatus, and answer immediately that this purpose is the pursuit of pleasurable excitement. It seems as if our entire psychological activity were directed toward gaining pleasurable stimulation, toward avoiding painful ones; that it is regulated automatically by the _principle of pleasure_. Now we should like to know, above all, what conditions cause the creation of pleasure and pain, but here we fall short. We may only venture to say that pleasurable excitation _in some way_ involves lessening, lowering or obliterating the amount of stimuli present in the psychic apparatus. This amount, on the other hand, is increased by pain. Examination of the most intense pleasurable excitement accessible to man, the pleasure which accompanies the performance of the sexual act, leaves small doubt on this point. Since such processes of pleasure are concerned with the destinies of quantities of psychic excitation or energy, we call considerations of this sort economic. It thus appears that we can describe the tasks and performances of the psychic apparatus in different and more generalized terms than by the emphasis of the pursuit of pleasure. We may say that the psychic apparatus serves the purpose of mastering and bringing to rest the mass of stimuli and the stimulating forces which approach it. The sexual instincts obviously show their aim of pleasurable excitement from the beginning to the end of their development; they retain this original function without much change. The ego instincts strive at first for the same thing. But through the influence of their teacher, necessity, the ego instincts soon learn to adduce some qualification to the principle of pleasure. The task of avoiding pain becomes an objective almost comparable to the gain of pleasure; the ego learns that its direct gratification is unavoidably withheld, the gain of pleasurable excitement postponed, that always a certain amount of pain must be borne and certain sources of pleasure entirely relinquished. This educated ego has become "reasonable." It is no longer controlled by the principle of pleasure, but by the _principle of fact_, which at bottom also aims at pleasure, but pleasure which is postponed and lessened by considerations of fact.