This chapter delves into the philosophical and psychoanalytic history concerning madness and its relation to knowledge. It dispenses with Lacan to begin with in order to track a line of philosophical thought within the continental tradition of which Lacan was very much a part. Utilizing a much-discussed debate between Foucault and Derrida on the nature of the cartesian cogito, I set out an argument that places madness and reason in close proximity. The Foucault-Derrida debate will allow us to construct a version of madness, much like a definition of paranoia, that is both ‘supplemental’ as well as being a necessary ‘internal excess’ to the practice of thought, philosophy or the cogito. Following this, the chapter introduces psychoanalytic contributions via the work of Althusser and Felman. Both provide theoretical tools, drawn from Freud, with which to decipher and interpret this ‘mad hyperbole’ at the heart of philosophical practice. Drawing on the idea that psychoanalysis is able to do something that the practice of reason (philosophy) cannot, precisely due to its ability to insert unreason (the unconscious) into the debate, we arrive at literature’s potential to encapsulate madness-in-language. I close by returning to Lacan’s structuralist period, in the guise of the Cahiers pour l’analyse, to establish what is meant by the ‘foreclosure of science’ and the fundamental affinity present between any system of knowledge and the structure of psychosis.