This thesis explores the theme of science in the short stories of the nineteenth-century Brazilian writer Machado de Assis, with an emphasis on ‘O Alienista’ (1882), ‘A Sereníssima República’ (1882), ‘Idéias de Canário’ (1899), ‘O Segredo do Bonzo’ (1882), ‘Conto Alexandrino’ (1884), ‘Verba Testamentária’ (1882) and ‘O Lapso’ (1884). With a particular interest in the many ways in which Machado deploys scientific theories as literary and sociological devices, this research examines how a broad concept of ‘unreason’ critiques modern state institutions such as hospitals, museums, prisons and government-sponsored medical projects. The critique extends to the self-professed liberal and politically independent scientific discourse in Brazil in the late nineteenth century, when the country was undergoing profound political, economic and social changes, especially those modes of discourse and political intervention promoted by an economically and racially hegemonic and homogenous intelligentsia. While Machado de Assis uses madness and egocentrism to challenge the idealistic view of the putatively reasonable and progressive ‘man of science’, he also denounces social tensions aesthetically through temporal and spatial displacements as well as fantastic elements, such as talking animals or absurd situations, which dialogue with the ancient tradition of the fable. The critical outcome is, thus, an ethically charged poetics of unreason which is a product of an ironic scepticism vis-à-vis Romantic nationalism, literary naturalism and scientific positivism, particularly as they bear on questions of race, gender, class and nationality and as disseminated by scientific institutions.