 The paper Hasle (2020) points out traces of hybrid-logical ideas in Prior’s works before the publication of Prior (1967) and Prior (1968) (new edition Prior (2003)). The above mentioned paper Øhrstrøm and Hasle (2006) gives a detailed account of Prior’s logical work. For a comprehensive account of Prior’s life and work, see the book Øhrstrøm and Hasle (1995). The paper Hasle and Øhrstrøm (2016) describes Prior’s methodological approach, in particular, his view on formalisation and the role of symbolic logic in conceptual studies.