The fact that the reality principle has to be re-established continually in the development of man indicates that its triumph over the pleasure principle is never complete and never secure.
The individual exists, as it were, in two different dimensions, characterized by different mental processes and principles. The difference between these two dimensions is a genetic-historical as well as a structural one: the unconscious, ruled by the pleasure principle, comprises “the older, primary processes, the residues of a phase of development in which they were the only kind of mental processes.” They strive for nothing but for “gaining pleasure; from any operation which might arouse unpleasantness (‘pain’) mental activity draws back.” But the unrestrained pleasure principle comes into conflict with the natural and human environment. The individual comes to the traumatic realization that full and painless gratification of his needs is impossible. And after this experience of disappointment, a new principle of mental functioning gains ascendancy. The reality principle supersedes the pleasure principle: man learns to give up momentary, uncertain, and destructive pleasure for delayed, restrained, but “assured” pleasure.
Eros and Civilization