The debate between realism and idealism continues to mark the discipline of International Relations. On the one hand, realism argues that international politics is a struggle for power and a quest for survival, which results in a condition of permanent conflict between States without any possibility of evolution or progress. On the other hand, idealism considers it possible to build a world of peaceful coexistence, prosperity and well-being, achieved through cooperation and based on values and aspirations shared by humans. The object of this article is to analyse the debate between idealism and realism, considering it as an ontological debate and taking into account the controversy it has generated. The argument presented here is that both realism and idealism are two responses to the creation and maintenance of international order, that is, how States relate in international society; however these responses are not mutually exclusive and can coexist in constant tension with one another. An analysis of internationalist thought of two authors, Hans Morgenthau and Raymond Aron, is also presented, which relates to how they are positioned in this debate as well as International Relations as a whole.