This article elaborates on Christopher Norris's claim that certain aspects of Derrida's work are amenable to formalisation in modal-logical terms. Norris contends that any adequate analysis of the logic behind Derrida's work must provide an account of the notions of possibility, necessity, and necessary possibility, particularly as they are related to Derrida's notion of iterability. This article examines the further hypothesis that Derrida's understanding of modality, according to which possibilities must be accounted for even if they are never realised, might (...) even better be described in terms of possible worlds. In possible-worlds semantics, the conceptual meaning of a statement is constituted by the set of alternative contexts in which that statement is true. This article argues, however, that possible-worlds semantics would be unthinkable without the experience that one referent can be substituted for another. The possibility of this experience is best described in Derridean terms. Read through Derrida's thought of the trace, the properly semantic substitutions may come to be seen as dependent on the substitution of the thing for itself; on what Derrida calls ‘the substitution of the unique for the unique’. (shrink)