Depressive position
In the depressive position, the infant is able to experience others as whole, which radically alters object relationships from the earlier phase. Before the depressive position, a good object is not in any way the same thing as a bad object. It is only in the depressive position that polar qualities can be seen as different aspects of the same object. Increasing nearness of good and bad brings a corresponding integration of ego. [...] In a development termed the "primal split," the infant becomes aware of separateness from the mother. This awareness allows guilt to arise in response to the infant's previous aggressive phantasies when bad was split from good. The mother's temporary absences allow for continuous restoration of her "as an image of representation" in the infant mind. Symbolic thought may now arise, and can only emerge once access to the depressive position has been obtained. With the awareness of the primal split, a space is created in which the symbol, the symbolized, and the experiencing subject coexist. History, subjectivity, interiority, and empathy all become possible. [...]
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In working through depressive anxiety, projections are withdrawn, allowing the other more autonomy, reality, and a separate existence. The infant, whose destructive phantasies were directed towards the bad mother who frustrated, now begins to realize that bad and good, frustrating and satiating, it is always the same mother [...]
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From this developmental milestone come a capacity for sympathy, responsibility to and concern for others, and an ability to identify with the subjective experience of people one cares about. With the withdrawal of the destructive projections, repression of the aggressive impulses takes place. The child allows caretakers a more separate existence, which facilitates increasing differentiation of inner and outer reality [...] When all goes well, the developing child is able to comprehend that external others are autonomous people with their own needs and subjectivity.
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Now some of you might say this 'breast' thing is all feasible ... it makes sense ... and yet, one might naturally pose the question: what about children who never had a female figure near during their childhood? What about, for instance, babies raised by gay partners (males) that we see more and more these days?
Klein put down in words her intuitions during a time when gay parenting and the like, e.g., were not even thought of - I'd guess, her theories need a bit revision. I don't want to sound rude, but speaking nowadays in her terms is a bit funny!