Luhmann defined social systems as structured around specific social codes, and comprised of the communications relating to that code. This thesis asks how the phenomenon of madness can be understood within this framework and argues that mad utterances are statements or acts which cannot be parsed according to any existing system of social or interpersonal meaning. The psy-disciplines transform these uncertain acts into stable meaning by defining them within a functionalist or pathological framework. These meanings are fragile because the operations of the psyche are socially invisible and so mad utterances have to be defined in relation to existing social systems of meaning. Mad utterances therefore generate uncertainty, which leads systems such as law and the economy to over-react to madness and discredit to a disproportionate degree what mad people have to say. The discreditation of mad people is problematic because it limits their personal autonomy. The ways in which systems exclude mad people, even when their stated objective is to promote their inclusion, is illustrated by the research literature on involvement in healthcare decision-making. The law plays a particular role in sustaining discreditable assumptions about mad people, and this is evidenced by a close examination of the research literature and case law relating to the Mental Health Review Tribunal in England and Wales. Luhmann's systems theory is not normative, so no clear normative agenda for change can be adduced from this description alone. Instead, this account offers a new theoretical framework within which to understand some of the shortcomings of mental health law, which is of particular relevance now that the involvement of mad people in decision-making them has been firmly placed on the legal and political agendas.