In probably each relation, there is a mixture of ingredients that its participants contribute to it alone and to no other, and of other ingredients that are not characteristic of it exclusively, but in the same or similar fashion are shared by its members with other persons as well. The peculiar color of intimacy exists if the ingredients of the first type, or more briefly, if the "internal" side of the relation, is felt to be essential

DEFINE: Introduces the 'internal side' of a relation as the mixture of ingredients exclusive to its participants, which constitutes intimacy.

Georg Simmel, The Sociology of Georg Simm…, loc. 3791