This paper aims to introduce a neglected methodology from Japanese international relations (IR) – the culturalist methodology – to Anglophone specialists in IR. This methodology is neglected not only by an Anglophone audience but also by Japanese IR scholars. I argue here that despite this negligence, the culturalist methodology has great potential to contribute to contemporary post-Western international relations theory (IRT) literature by posing radical questions about the ontology of IR, as it questions not only the ontology of Western IR, but also the IR discourses developed in the rest of the world. Consequently, in understanding and imagining the contemporary world, I clarify the importance of perceptions based on what, in Japan, are commonly called ‘international cultural relations’ (kokusai bunka) and ‘regional history’ (chiikishi). I also indicate how our perceptions of the world are limited by the Westphalian principles of state sovereignty and non-intervention among ‘equal’ nations on the basis of state borders. While historical understanding is widely recognised as an important approach to contemporary IR, its scope is limited by its universalised principles.