When Jean Hyppolite asks: "What use does the Symbolic have?" Lacan answers: "The Symbolic, the Imaginary and the Real are useful in giving its meaning to a particularly pure symbolic experience, that of analysis." Since the symbolic dimension is the only dimension that cures, "The symbolic order is simultaneously non-being and insisting to be, that is what Freud has in mind when he talks about the death instinct as being what is most fundamental: a symbolic order in travail, in the process of coming, insisting in being realised."
A four-term structure maps the Real, the Imaginary and the Symbolic as replacing the second Freudian topography: ego/id/superego. Two diagonals intersect, while the imaginary rapport links a (the ego) to a' (the other), the line going from S (the subject, the Freudian id) to A
(the Other) is interrupted by the first one. The Other is difficult to
define: it is the place of language where subjectivity is constituted;
it is the place of primal speech linked to the Father; it is the place
of the absolute Other, the mother in the demand. The Other makes the
subject without him knowing it.
Le séminaire, Livre II: Le moi dans la théorie de Freud et dans la technique de la psychanalyse