Klein's theory grew out of her technique. In The Psychological Principles of Infant Analysis1 she argued that play expresses the child's phantasies symbolically, as dreams do, and can be interpreted like free association. She also argued that Oedipal tendencies and a strict super-ego appear far earlier than had been assumed. Looking back in The Psycho-Analytic Play Technique20, she described several elements of the method: the playroom, the child's own set of small toys, and interpretation that includes the negative transference. She also described how the technique led her to her theory of early anxieties, internal objects and positions.
Personification in the Play of Children21 showed that the characters children invent in play reduce to a few roles: a persecuting super-ego, the id or ego, and a helping figure. The super-ego, she argued, is split into its identifications and projected outwards; this is the first use of splitting in her work. The analyst has to accept every role offered, including the hostile ones.
In Early Stages of the Oedipus Conflict5 she dated the onset of the Oedipus conflict and of the super-ego to the frustrations of weaning towards the end of the first year, when oral and anal sadism are at their height. She proposed a "femininity phase" in both sexes, and traced the epistemophilic instinct to curiosity about the mother's body. She pursued that theme in A Contribution to the Theory of Intellectual Inhibition22. In The Psycho-Analysis of Children the early super-ego is harsh and pregenital, against Freud's view that the super-ego is the heir of the Oedipus complex.
The analysis of Dick, a four-year-old whose symbol formation had almost come to a halt, led her to argue that anxiety drives symbol formation by moving interest on to new objects (The Importance of Symbol-Formation6). In Infantile Anxiety-Situations Reflected in a Work of Art and in the Creative Impulse23 the wish to make good the injury done to the mother first appears. That wish later became her concept of reparation.
The 1935 psychogenesis paper8 introduced the depressive position, which she dated to around the middle of the first year. The infant comes to experience the mother as a whole person who is both loved and hated, rather than as split good and bad part-objects, a notion taken from Abraham's idea of partial love. This brings depressive anxiety and guilt for the loved object, and the wish to make reparation. The paper also described the manic defence, made up of omnipotence, denial and triumphant control. It set reparation within the depressive position and distinguished it from manic reparation.
Mourning and its Relation to Manic-Depressive States10 extended this account to mourning. Normal mourning, she argued, is a modified and transitory manic-depressive state. A real loss revives the fear of having lost the good internal objects, and mourning succeeds by reinstating them; the infantile neurosis is the means by which the depressive position is worked through. Triumph, control and contempt obstruct this Working through. She saw the depressive position as never finally achieved: it is reworked throughout life and alternates with the paranoid-schizoid position.
The 1945 paper on the Oedipus complex, drawing on the cases of Richard and Rita, revised her earlier account in the light of the depressive position. It held that love and the wish for reparation, as well as sadism, shape the early Oedipus situation, and ended with a point-by-point comparison with Freud.
In Notes on Some Schizoid Mechanisms14 she described the paranoid-schizoid position of the first three or four months. At this stage an ego that exists from birth, but lacks cohesion, meets persecutory anxiety rooted in the death instinct and felt as a fear of annihilation. It defends itself by splitting both the object and itself, and by idealization, denial and omnipotence. In projective identification, split-off parts of the self, good as well as bad, are projected "into" the mother rather than merely "on to" her, so that she is identified with them; in excess this leaves the ego depleted. The introjected good breast becomes a focal point of the ego. She placed the fixation points of schizophrenia in this position and those of manic-depressive illness in the depressive one, and held that normal splitting is a forerunner of repression. She took the word "schizoid" from Fairbairn, hence "paranoid-schizoid", while rejecting his neglect of anxiety, aggression and the death instinct. The term projective identification appears once in the 1946 paper; the sentence explicitly proposing it as a name was added in the version revised for Developments in Psycho-Analysis.
A Contribution to the Theory of Anxiety and Guilt24 set the two anxieties side by side, against Freud's late theory of anxiety: persecutory anxiety is fear for the self, depressive anxiety is fear and guilt for the object. Some Theoretical Conclusions (1952) gave her most systematic account of the first year. The Origins of Transference25 rooted transference in the earliest object relations and treated it as a "total situation" carried from the past into the present.
In Envy and Gratitude19, her last major theoretical work, she proposed a primary envy of the good, feeding breast, distinguished it from greed, and paired it with gratitude as an expression of the life instinct.