I need to read Poul F. Kjaer's Systems in Context: On the Outcome of the Habermas/Luhmann-Debate (Ancilla Iuris, Vol. 66, 2006) which bears this abstract:
Usually regarded as a 1970s phenomenon, this article demonstrates that the debate between Jürgen Habermas and Niklas Luhmann continued until Luhmann’s death in 1998, and that the development of the two theorists’ positions during the 1980s and 1990s was characterised by convergence rather than by divergence. In the realm of legal theory, the article suggests, convergence advanced to the extent that Habermas’ discourse theory may be characterised as a normative superstructure to Luhmann’s descriptive theory of society. It is further shown that the debate’s result was an almost complete absorption of Habermas’ theory by Luhmann’s systems theoretical complex — an outcome facilitated by Luhmann’s deliberate translation of central Habermasian concepts into systems theoretical concepts. The article argues that both the debate and Habermas’ conversion were made possible because not only Habermas’ but also Luhmann’s work can be considered a further development of the German idealist tradition. What Luhmann did not acknowledge was that this manoeuvre permitted the achievement of Habermas’ normative objectives; nor did he notice that it could eradicate a central flaw in the system theoretical construction, by allowing the context within which distinctions are drawn to be mapped — an issue of pivotal importance for grasping relationships between different social systems, and coordinating them, via the deployment of legal instruments.