Complex sentence: Luhmann, in contrast, treats his vision of systems ontologically, saying that "systems exist", that is Luhmann in fact suggests to change the ontological paradigm with the paradigm of systems theory: the difference system/environment (which also signifies a relationship).
Split into simpler sentences: Luhmann, in contrast, treats his vision of systems ontologically, saying that "systems exist". That is, Luhmann in fact suggests to substitute the paradigm of systems theory for the ontological paradigm: the difference system/environment (which also signifies a relationship).