On the basis of research into the didactics of biology, the article presents some general reflections on the common object of subject didactics. Starting from a controversy in science (in ecology), it foregrounds the problem of the choice of contents to be taught. The theoretical framework of the research study is the Model of Didactic Reconstruction conceived for research into subject didactics. The model is constructed on three mutually complementary tasks: clarification of the pupil‘s ideas, clarification of the scientist‘s ideas and the creation of methods of teaching. The ways in which pupils and scientists understand ecological terms in epistemologi-cal and ontological contexts is central, and constructivism is not taken as a theoretical premise but is re-evaluated through the analysis. On the basis of reflection based on the analysis of the tasks in the model, the goal of teaching is presented as „critical knowledge“ - i.e. knowledge which involves conscious thought on the contradictions and principles involved in the ideas concerned. The creation of meaningful teaching material is the common aim of subject didactics.