The first case of such a check on the pleasure-principle is perfectly familiar to us in the regularity of its occurrence. We know that the pleasure-principle is adjusted to a primary mode of operation on the part of the psychic apparatus, and that for the preservation of the organism amid the difficulties of the external world it is ab initio useless and indeed extremely dangerous. Under the influence of the instinct of the ego for self-preservation it is replaced by the ‘reality-principle’, which without giving up the intention of ultimately attaining pleasure yet demands and enforces the postponement of satisfaction, the renunciation of manifold possibilities of it, and the temporary endurance of ‘pain’ on the long and circuitous road to pleasure. The pleasure-principle however remains for a long time the method of operation of the sex-impulses, which are not so easily educable, and it happens over and over again that whether acting through these impulses or operating in the ego itself it prevails over the reality-principle to the detriment of the whole organism. It is at the same time indubitable that the replacement of the pleasure-principle by the reality-principle can account only for a small part, and that not the most intense, of painful experiences. Another and no less regular source of ‘pain’ proceeds from the conflicts and dissociations in the psychic apparatus during the development of the ego towards a more highly co-ordinated organisation. Nearly all the energy with which the apparatus is charged comes from the inborn instincts, but not all of these are allowed to develop to the same stage. On the way it over and again happens that particular instincts, or portions of them, prove irreconcilable in their aims or demands with others which can be welded into the comprehensive unity of the ego. They are thereupon split off from this unity by the process of repression, retained on lower stages of psychic development, and for the time being cut off from all possibility of gratification. If they then succeed, as so easily happens with the repressed sex-impulses, in fighting their way through—along circuitous routes—to a direct or a substitutive gratification, this success, which might otherwise have brought pleasure, is experienced by the ego as ‘pain’. In consequence of the old conflict which ended in repression the pleasure-principle has been violated anew, just at the moment when certain impulses were at work on the achievement of fresh pleasure in pursuance of the principle. The details of the process by which repression changes a possibility of pleasure into a source of ‘pain’ are not yet fully understood, or are not yet capable of clear presentation, but it is certain that all neurotic ‘pain’ is of this kind, is pleasure which cannot be experienced as such.