This paper aims to map the possibilities of adopting social systems theory in order to couple three usually isolated domains: social theory, jurisprudence and empirical research. It argues that these different uses would be the distinctive legacy of Luhmann’s work for legal research. The text suggests that investigations interested in adopting a systemic approach for discussions in each of these three domains (or in all of them) should focus on the entanglement among interactional, decisional and functional systems. However, this presupposes some enhancements in Luhmann’s own description of law as a social system. The core hypothesis presented here is that a functional system’s out-differentiation (its specialisation in face of other communications happening in the societal environment) depends on the inner-differentiation of this system – that is, on operations backed by the very construction of specific functional-systemic institutions and semantics (including decisional programs and self-descriptions).