The proto-mental system I visualize as one in which physical and psychological or mental are undifferentiated. It is a matrix from which spring the phenomena which at first appear—on a psychological level—to be discrete feelings only loosely associated with one another.
Bion defines the proto-mental system as the undifferentiated somatic-psychic matrix from which basic-assumption emotions—and by implication any proto imaginal activity—arise prior to psychological differentiation.
, Experiences in Groups and Other Papers, 1959thesis