  Prior introduced the collective term ‘quasi-modals’ for the non-alethic modes (1951: 749) and remarked, accurately, that ‘there is a hint of a large field here’ (1951: 752). He was later to refer to his own tense operators as quasi-modal operators (1968: 138). By the time he wrote Formal Logic he was advocating the study of ‘the general modal form “It is — that \(p\)” … as a distinct propositional form’, observing that ‘this field has not been much cultivated’ (1955a: 218).