Fusion is one of the fundamental mechanisms of mental functioning, an essential element in all object relations - with variations only in the degree of participation - along with the component connoted by separateness. There is a continuous dialectic relationship between levels of fusion and separateness in every human relationship.

Among the goals of the basic human search for an object we should also include the attempt to establish common mental areas with sufficiently similar objects through fusion. Mental contents can flow freely between the subject and the objects through these shared areas. Human beings seem characterized by an extremely sophisticated and continuously changing boundary system.

The simultaneous dialectic presence of three levels of mutual relation is stressed in object relations: the level of fusion, the level of incomplete separation and the level of complete separation. The three levels are compared to the three positions (one more primitive: Fusion as well as the Kleinian PS and D).