The authors explore the interpersonal aspects of the early development of an experience of external reality and the roots of this experience in primary intersubjectivity. They suggest some implications that this has for psychoanalytic work with the patient's experience of external reality. They argue that the external world is not an independently existing 'given', for the infant to discover, as is sometimes implicitly assumed. Infants acquire knowledge about the world not just through their own explorations of it but by using other minds as teachers. The experience of external reality is invariably shaped through subjectivities. The authors argue that at first the infant assumes that his knowledge is knowledge held by all, that what he knows is known by others and that what is known by others is accessible to him. Only slowly does the uniqueness of his own perspective differentiate so that a sense of mental self can develop. In clinical work we frequently observe the undoing of this process of differentiation, and understanding the underlying mechanisms can be helpful in managing the transference and countertransference consequences when the process has been derailed.