There is a long tradition of opposing the passions to reason, and discussing their harmful effects. As Foucault points out, the link between passion and madness was well-established. But Foucault goes further by arguing that a theory of the passions which links body and soul helps to create something like madness, which affects both body and soul. Foucault automatically assumes that madness affects body and mind, even though very few passion theorists argue about madness in these terms.

Foucault's analysis of classical madness and passion deals with the effects of madness on the body. But what interests him more are the products of the insane mind: delirium and hallucinations. He examines the insane mind by contrasting madness with two similar states, imagination and dreaming. Both involve strange and unreal images that are similar to those seen and experienced by madmen.

However, for Foucault the key difference between madness and these unreal images is truth. Foucault says that madness exists when someone believes that fantastic images are true. Madness involves a distortion of the truth as the mad person experiences it. Foucault stresses the coherence and logic of madness. It has its own language, and the delusion of the madman makes sense within his distorted world. While delirium is a symptom of certain kinds of mental disease, Foucault identifies a different form of delirium, which he calls a discourse.

Discourse is a central concept for Foucault. He first introduces the concept in Madness and Civilization. A discourse is essentially a total system of knowledge that makes true or false statements possible. The madman believes unreal things to be true because the discourse that structures his belief dictates it. In Foucault's later work, he examines the discourses of psychiatry, medicine, and sexuality. Here, he emphasizes the unreal but powerful nature of delirium. Delirium structures perception and truth and makes "unreal" beliefs possible.

Delirious discourse is a phenomenon of language and belief, but it affects the body and mind. Foucault is clear that delirious discourse does not originate in the mind or body. The idea of discourse and truth underlies all the different types of madness that the classical period identified. Deliria, dementia and hallucinations were seen as different types of madness, but in fact they all relate to truth and to a distorted discourse.

Blindness shows the relationship of madness to reason. The madman sees the same reason as the sane man, but in a different way. This is also the first appearance of Foucault's analysis of Descartes. Descartes's argument against skepticism about his own existence and sanity, the Cogito, is seen by Foucault as evidence of the self-assurance of classical reason. For Descartes, the fact that he doubts proves his own existence, and proves that he cannot be mad. Essentially, his reason denies the possibility of madness. Foucault's interpretation of Descartes is one of the most controversial sections of this work, and provoked a controversy with the philosopher Jacques Derrida.

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