Sign

The Sign, as Lacan deploys it in Seminar XX, is a semiotic unit that is rigorously distinguished from the signifier. Where the signifier is what represents the subject for another signifier — operating entirely within the Symbolic register and introducing the One into the world — the sign is defined by its relation to an effect rather than to a thing. Lacan's formula is precise: "a sign is not the sign of some thing, but of an effect that is what is presumed as such by a functioning of the signifier." The sign therefore presupposes the signifier's prior operation; it marks the trace or residue left by that operation — not the signifying chain itself, but the presumed subject-effect that the chain generates. This is why desire (not love) is aroused by the sign of the subject: the sign indexes the subject as an effect sliding between signifiers, catching desire at the level of its cause (objet petit a) rather than at the level of symbolic identification.

In the second occurrence, the sign receives a further, ontological characterization: it is "defined only from the disjunction of two substances, that have no part in common." This formulation places the sign at the boundary of the Real — at a point of radical non-relation or disjunction — rather than in the articulated difference that constitutes the signifier's Symbolic domain. It is against this background that Lacan's enigmatic statement "Love is the sign indicated as such that one is changing reason" makes structural sense: love, which aims at the subject as such rather than at its signifying effects, is a sign of a change of discourse (a shift in the social bond), not a manifestation of the Other's jouissance. The sign thus operates at the intersection of Symbolic effect and Real disjunction, marking the limit-point where the signifier's work generates something that cannot be re-absorbed into the chain.

Both occurrences of Sign appear in Seminar XX (jacques-lacan-seminar-20-bruce-fink and jacques-lacan-seminar-20-cormac-gallagher), Lacan's most sustained meditation on jouissance, love, and the sexual non-relation. Within that seminar's argument, the Sign functions as a conceptual hinge between the Signifier (whose canonical definition is "what represents a subject for another signifier") and Desire (whose cause is objet petit a). The sign is not itself a signifier; it presupposes the signifier's operation and marks only the presumed effect — the subject-as-effect — that results from it. This means the sign sits at the interface of the Symbolic and the Real: it belongs to the register of desire and jouissance rather than to the purely Symbolic economy of the four discourses, yet it is also the operator through which a change of discourse (a shift among the Four Discourses) can be registered as Love. In this sense the Sign is a specification and a limit-concept relative to the Signifier: it captures what cannot be fully symbolized but can nonetheless be indicated — the subject's residual effectivity and the disjunction of two incommensurable substances. Its relation to Jouissance is equally precise: the sign does not deliver jouissance but marks its boundary, indexing the point where the signifier's productivity meets the Real of non-relation. The Sign thus extends the canonical apparatus of Language and Signifier while carving out a distinct function irreducible to either pure Symbolic difference or raw Real enjoyment.

Seminar XX · Encore: On Feminine Sexuality, the Limits of Love and KnowledgeJacques Lacan · 1972 (p.63)

The sign is what is defined only from the disjunction of two substances, that have no part in common... Love is the sign indicated as such that one is changing reason.

The phrase "disjunction of two substances, that have no part in common" is theoretically charged because it locates the sign at the Real — at a point of absolute non-relation — rather than in the differential articulation that defines the Symbolic signifier; and the identification of Love as precisely such a sign of "changing reason" (changing discourse) ties the ontological claim directly to Lacan's theory of the Four Discourses, making the sign the operator that marks a structural shift in the social bond itself.