Lacan presented his work as a return to Freud and himself, in Johnston's description, as the lone defender of a Freudian orthodoxy in danger of being eclipsed by the post-Freudians. His central thesis, that the unconscious is structured like a language, speaks of un langage, the logic of syntax and meaning in general, and not of une langue, a particular natural language; whatever he later revised in the comparison, he held that the unconscious is structured and not a chaotic id. The papers collected in Écrits1 turn this thesis against ego psychology, arguing that to strengthen an autonomous and adapted ego, as Hartmann, Kris and Loewenstein proposed, betrays Freud's discovery of the unconscious.
Johnston treats three registers, the Imaginary, the Symbolic and the Real, as the framework of most of Lacan's thought. Lacan revised them repeatedly, and in the 1970s figured their mutual dependence as a Borromean knot, in which breaking one ring sets all three free. The Imaginary is the register of the ego and of the images subjects form of themselves and of others, and is central to the mirror stage. The Symbolic is the order of language, law, custom and ritual into which subjects are born, the big Other. The Real is what resists symbolization, the limit of the other two registers. To these Lacan added objet petit a, the object-cause of desire, which makes empirical objects desirable while none of them matches it. Scholars sometimes divide his teaching into an early period dominated by the Imaginary, a middle one by the Symbolic and a late one by the Real, a periodization that Johnston says should be taken with several grains of salt.
In "The Instance of the Letter in the Unconscious" (1957, collected in Écrits1), Lacan turned Jakobson's distinction between the metaphoric and metonymic poles of language into a psychoanalytic one. Metaphor is the substitution of one signifier for another in the signifying chain, the hidden signifier remaining linked to the rest of the chain, and is the structure of condensation and of the symptom. Metonymy is the connection of signifier to signifier along the chain, and is the structure of displacement, by which the unconscious gets round censorship. Hence his formula that the symptom is a metaphor and desire a metonymy. In Seminar XI he restated condensation as a signifying operation with the effect of metaphor, referring back to the 1957 paper (The Four Fundamental Concepts of Psychoanalysis4).
The Name-of-the-Father, first named in the Rome discourse as the support of the symbolic function, identifies the father with the law. In his 1958 essay on the treatment of psychosis Lacan cast it in the form of his metaphor: the Name-of-the-Father takes the place of the mother's desire, and the substitution produces the phallic signification. When this signifier is foreclosed and the paternal metaphor fails, psychosis follows; Lacan held this to be the essential condition of psychosis and what sets it apart from neurosis (Écrits1). From 1963 he spoke of the Names-of-the-Father in the plural.
Lacan reworked a number of clinical concepts in the same terms. Against an analysis of resistances directed at the patient's ego, he located resistance on the analyst's side, as a product of the analyst's own pressure and demand, and read the subject's resistance to suggestion as the wish to keep desire alive (Écrits1). In Seminar XI he located transference in the analyst's position as the "subject supposed to know". He took over Jones's term aphanisis, turning it from the loss of desire into the fading of the subject, who disappears beneath the signifier that represents it. He grouped the parapraxis with the dream and the joke as the formations in which the unconscious first shows itself, as a stumbling or a crack in what is said, and reread Freud's forgetting of the name Signorelli as the disappearance of the signifier "Signor", the absolute master, which he linked to death. He also objected that analysts handle projection and introjection loosely, as if they were simply the reverse of each other, when one belongs to a field dominated by the Symbolic and the other to the Imaginary (The Four Fundamental Concepts of Psychoanalysis4).
In the seminar of 1959–1960, published as The Ethics of Psychoanalysis5, Lacan read Freud's das Ding as the lost and inaccessible Thing around which desire is organised. He moved sublimation away from a change in the drive's aim toward the place of the object: sublimation raises an ordinary object to the dignity of the Thing, and art is organised around the void that the Thing leaves. The same seminar reads Antigone and asks what an ethics of desire would be for psychoanalysis. Johnston notes that Seminar VII is often taken to mark a reorienting turn toward the Real, although the claim of such a turn remains controversial. In Seminar XVII (1969–1970) and the interview "Radiophonie", Lacan set out his theory of the four discourses: those of the master, the university, the hysteric and the analyst.