Three Passages from Freud
… I must ask you to picture the ego as a kind of façade of the id, as a frontage, like an external, cortical, layer of it. We can hold on to this last analogy. We know that cortical layers owe their peculiar characteristics to the modifying influence of the external medium on which they abut. Thus we suppose that the ego is the layer of the mental apparatus (of the id) which has been modified by the influence of the external world (or reality). This will show you how in psychoanalysis we take spatial ways of looking at things seriously. For us the ego is really something superficial and the id something deeper — looked at from outside, of course. The ego lies between reality and the id, which is what is truly mental. (From The Question of Lay Analysis)
Well, then, we assume that the forces which drive the mental apparatus into activity are produced in the bodily organs as an expression of the major somatic needs. … We give these bodily needs, in so far as they represent an instigation to mental activity, the name of “drives,” a word for which we are envied by many modern languages. Well, these drives fill the id: all the energy in the id, as we may put it briefly, originates from them. Nor have the forces in the ego any other origin; they are derived from those in the id. What, then, do these drives want? Satisfaction — that is, the establishment of situations in which the bodily needs can be extinguished. A lowering of the tension of need is felt by our organ of consciousness as pleasurable; an increase of it is soon felt as unpleasure. From these oscillations arises the series of feelings of pleasure-unpleasure, in accordance with which the whole mental apparatus regulates its activity. In this connection we speak of a “dominance of the pleasure principle.” (From The Question of Lay Analysis)
Nor does the isolated instinctual impulse remain idle; it understands how to make up for being denied normal satisfaction; it produces psychical derivatives which take its place; it links itself to other processes which by its influence it likewise tears away from the ego; and finally it breaks through into the ego and into consciousness in the form of an unrecognizably distorted substitute, and creates what we call a symptom. (From The Question of Lay Analysis)