International Relations is uncertain about its status as a „science” and as a rational enterprise capable of producing knowledge about the world of international politics. Throughout a long disciplinary history of attempts to legitimate the field as „scientific”, International Relations scholars have imported many positions from Philosophy of Science in order to ground International Relations on an unshakable foundation. Philosophical questions are commonly seen as universal, timeless, and abstract in nature. As for Philosophy of Science, it is conceived to involve the study of abstract questions of logic, epistemology, and ontology, specifically in relations to how scientific claims are justified or structured. Alas, no such unshakable foundation exists. The Philosophy of Science is itself a contested field of study, in which no consensus exists on the proper foundation for science. There are at least three well-supported foundational positions: Instrumentalism, Social Constructivism and Scientific Realism. None of them has produced consensus among philosophers. In this article, author presents tensions between different Philosophy of Science and International Relations Theory. The „science” debates in International Relations has divided the discipline on the possibility of a science international relations. Foundational positions have become part and parcel of the way International Relations scholars think about their scientific work. The ongoing division among positivists, anti-positivists, and post-positivists is the inevitable result of each side’s claim to represent the right position in the Philosophy of Science.