Four levels of object relationships preceding the achievement of object constancy are defined: biological unity between infant and mother; patterned activity, corresponding to autoerotism and including rocking, whirling, circular motions, by which the child tries to maintain a sense of familiarity and comfort; part-object representation, including imitative behavior, marking the emergence of self-object differentiation; and mutative-object representation, in which the child first makes a cognitive bridge between two part-objects.