 Be that as it may, what this brief account of the history of the historical challenge to realism reveals are the two major lines of defense of realism at play. Both lines of defense are based on the presence of substantial continuity in theory-change in the history of science. This continuity suggests that the disruption of the scientific image of the world, as theories change, is less radical than is assumed by the historical challenge to realism. But the two lines of defense (the Poincaré-Duhem and the Boltzmann one) disagree over what is retained when theories change. The Poincaré-Duhem line of defense focuses on mathematical equations (which express relations) and claims that only relations among unobservable things are knowable, whereas the Boltzmann line of defense focuses on whatever theoretical elements (including entities like atoms) are retained while theories change; hence, it does not limit scientific knowledge to the knowledge of relations only. Both lines have resurfaced in the current debate.