This article discusses the relevance of the systems theoretical distinction between "medium" and "form" as a tool of scientific analysis in the social sciences. It is an attempt to construct a medium theory that remains true to George Spencer-Brown's logical calculus on the one hand, and that gives full "credit" to Niklas Luhmann's considerations on the medium/form distinction on the other hand. The theoretical usefulness of the distinction is demonstrated by means of the exemplary medium of money. The analysis shows the growing centrality of the medium/form distinction in Luhmann's epistemology to the point of becoming the bedrock of his constructivist theory of social systems in his later works.