Klein stresses the all-pervasive power of (and struggle between) the life and death instincts; the need to master anxiety arising from this struggle leads to ego function 'at least from birth onwards'. The primordial anxiety which the ego fights is the threat arising from the death instinct, the mechanism of projection protecting one from self-destructive drives. Beginning at birth both instincts are attached to objects, the breast first. All processes of internalization derive from introjection of the good (feeding) breast. Both the 'good' and the (projected) 'bad' breast are introjected, and thus can operate from within the ego. The strength of the ego is 'constitutionally determined' but affected also by external factors, and reflects the state of fusion between the two instincts. It is the complexity of the fluctuations in the constant activity of the two instincts that underlies the development of the ego. Klein discusses the origins of the superego, the clinical implications of these hypotheses in the neurotic and the psychotic, and other aspects.