One of Prior’s conclusions in ‘Modality’, significant for his later work, is that ‘[t]here is everything to be said … for the … view that we may not only use devices developed in the study of quantity to throw light on modality, but also vice versa’ (1951: 747). One of the most distinctive features of his mature philosophy was the view that quantification over possible worlds and instants is to be interpreted in terms of modality and tense, which constitute primitive notions—a view he held in tandem with the belief that the study of such quantifications could usefully illuminate the study of modality and tense (as in his own \(U\)-calculi, described below).