In this paper, through clinical example, I have attempted to illustrate and to delineate aspects of the pathology of the separation–individuation process as they are manifested in later clinical work. I have distinguished between pathology of the relation to the differentiated other and the undifferentiated other, the latter being the primary phenomena with roots in the separation–individuation process; and I have attempted to show that the confusion between these two rests on a focus on surface presentations rather than in-depth understanding. A further distinction was drawn within the pathology of the relation to the differentiated other; some clinical phenomena there can be seen as tied to the differentiation process itself, and are thus also linked to separation–individuation. Finally, the relevance of the concepts of separation–individuation to depersonalization/derealization, to 'as if' character, and to folie à deux was discussed.