This book is a close reading of Jacques Lacan’s seminal essay, The Agency of the Letter in the Unconscious or Reason Since Freud, selected for the particular light it casts on Lacan’s complex relation to linguistics, psychoanalysis, and philosophy. It clarifies the way Lacan renews or transforms the psychoanalytic field, through his diversion of Saussure’s theory of the sign, his radicalization of Freud’s fundamental concepts, and his subversion of dominant philosophical values. The authors argue, however, that Lacan’s discourse is marked by a deep ambiguity: while he invents a new language, he nonetheless maintains the traditional metaphysical motifs of systemacity, foundation, and truth.