  Prior wished to formalise the ancient insight that propositions can change in truth value with the passage of time. He soon realised that off-the-shelf modal syntax could be adapted to do this. It was simply a matter of taking seriously the idea he had discussed in The Craft of Formal Logic: tense is a species of modality, to be set alongside the ordinary (‘alethic’) modes of necessity and possibility. When he discovered the ‘Master Argument’ of Megarian logician Diodorus Chronos (in Benson Mates’ 1953 book Stoic Logic), Prior’s exegesis of Diodorus’s argument became the backdrop to his first publication on tense logic, ‘Diodoran Modalities’ (1955b).