This article describes the origins and some of the history of the Welsh-language philosophy journal Efrydiau Athronyddol, which was published from 1938 to 2006. The majority of the papers published in the journal were presented at the annual conference of the Welsh-speaking Philosophy section of the Guild of Graduates (Adran Athroniaeth Urdd y Graddedigion), an annual conference which continues today but which was established in the early 1930s. The article describes the nature and content of the first issue, and provides a summary of some of the main themes of articles published. It shows how the character of the journal underwent a significant change in 1949 as a result of a deliberate policy on the part of those most closely involved with the running of the journal and indeed in the Philosophy section of the Guild. The journal subsequently changed from being a purely philosophical one to a more interdisciplinary publication which dealt with a much wider range of topics, many of which had a distinct focus on Welsh intellectual life. The second half of the article focuses on one of the most influential papers ever published in the Efrydiau, namely ‘The idea of a nation’ by Professor J. R. Jones. Published the year before Saunders Lewis’s radio broadcast ‘Tynged yr Iaith’, its main claims are described and subjected to critical analysis. This paper exemplifies what was best about the journal: it is philosophical, but also interdisciplinary – drawing on poetry and history – and makes powerful political claims which led to Jones being described by Professor D. Z. Phillips as the philosophical inspiration for the Welsh Language Society, in addition to an acknowledged influence on the thinking of Saunders Lewis.