The article belongs to the so-called "theories of complexity" and to the attempts to apply them to sociology. It compares the theoretical points of view of Edgar Morin and of the Spanish sociologist Jesús Ibáñez, which belong to the "paradigm of complexity", analyzing their confluences and divergences, seeking the development of a "complex sociology". Starting from a comparative approach, the article suggests some possible keys and orientations to build a "complex social science", which integrates ontology, epistemology, and ethics, seeking also to open, with a critical thinking, to current complexity modellings, such as agent-based modelling and simulation.