Pohl and Hirsch Hadorn establish one of the clearest early operational grammars for transdisciplinary research. Their decisive contribution is to define transdisciplinarity not as disciplinary mixture but as a recursive process structured around complexity reduction, contextualisation, open encounters and reflexivity. The method is explicitly designed to connect scientific inquiry with life-world problems, distinguishing systems knowledge, target knowledge and transformation knowledge while repeatedly reformulating research questions in relation to actors and contexts. Recursiveness is central: problem definition, analysis and implementation are not successive sealed stages but mutually correcting moments. The work therefore gives transdisciplinarity an architectural structure in which participation, integration and contextual embedding can be designed rather than left to goodwill. Its importance extends beyond sustainability research because it offers a general model for research fields in which epistemic plurality, societal relevance and methodological openness must coexist. It bridges science studies, systems thinking and participatory research by making reflexivity itself an operational principle.