In the literature investigating the long history of appeals to ‘nature’, in its multiple meanings, for rules of conduct or justification of social order, little attention has been paid to a long-standing tradition in which medical and physiological arguments merged into moral and social ones. A host of medical authors, biologists, social writers and philosophers assumed that nature spoke its moral language not only in its general economy, but also within and through the body. This is why, for instance, many (...) critics of Malthus argued that physiological self-regulating mechanisms ensured a spontaneous adaptation of fertility to the circumstances. Beliefs in a beneficent economy of nature persisted when Providence was replaced by evolution. To some, they provided reasons for hope even when others worried about ‘unfit’ members of society increasing at a quicker rate than ‘fit’ ones. The nerves gradually replaced food and blood as moral mediators between society and the body. When faith in bio-social progress was shaken by degeneration theories, and fin de siècle anxiety concerned underpopulation rather than overpopulation, not an insignificant proportion of those who emphasised the bad effects of modern civilisation resorted to ideas and arguments based on a view of the body in which physiology and morality worked together. A common language and common assumptions linked the Kulturpessimisten to their optimistic eighteenth-century colleagues, in spite of their different forecasts. The present paper traces continuities and discontinuities in the operation of a persistent set of interrelated ideas and assumptions on the coalescence of the physiological and the moral across disciplines and contexts. (shrink)