Then, following the catastrophic discovery of otherness, comes the equally traumatic discovery of the difference between the sexes.
McDougall, J. (2000). Sexuality and the Neosexual
I think this statement can be related to Freud's work in two basic ways
Whatever the character's later capacity for resisting the influences of abandoned object-cathexes may turn out to be, the effects of the first identifications made in earliest childhood will be general and lasting. This leads us back to the origin of the ego ideal; for behind it there lies hidden an individual's first and most important identification, his identification with the father in his own personal prehistory. (Freud, The Ego and The Id p. 31).
I think that it's the confrontation with otherness which makes us identify with the father in the capacity of symbol formation
In that phase of children's libidinal development which is characterized by the normal Oedipus complex we find that they are tenderly attached to the parent of the opposite sex, while their relation to the other parent is predominantly hostile. In the case of boys the explanation is simple. A boy's mother was his first love-object; she remains so, and, as his feelings for her become more passionate and he understands more of the relation between father and mother, the former inevitably appears as a rival. With little girls it is otherwise. For them, too, the mother was the first love-object…. I have begun by stating the two facts which have struck me as new: first, that the great dependence on the father in women merely takes over the heritage of an equally great attachment to the mother and, secondly, that this earlier phase lasts longer than we should have anticipated (Freud, Female Sexuality, p.281, 283).
In both the case of the encounter with otherness and difference between the sexes I believe the child is reacting against the primary caregiver (i.e. the mother) and the father is a negative creation (since children from single mother's don't necessarily have children with mental illness, or as Lacan puts it, the father is normative and not natural).
Two important splits in the psyche occur in these two moments and they would correspond to what Lacan wants to call the ideal ego and the ego ideal.
I think that the encounter with othersness is the early oedipus complex of Melanie Klein's work.
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