This volume contains a series of articles which promote, each in a different way, a multi-disciplinary conceptual engagement with Niklas Luhmann's writings on sociological method. Each article illuminates diverse threads of possible philosophical debate around the Luhmannian theory of society, and Luhmann's ambiguous location at once inside and outside different strands of the European philosophical tradition comes distinctively and variably into focus throughout all the articles. In particular, consideration is given to implications of his thinking for fundamental ontology, the critique of epistemology, post-structuralism, the philosophy of difference, philosophical functionalism, the general philosophy of the social sciences, and the methodological relations between philosophy and sociology. Running through the chapters is concern with the theme of de-ontologization. Each essay clarifies how his thought, although critical of established positions in the philosophical canon, originally developed through an intense engagement with the legacy of classical European thinking.