  For the similarity in behaviour between signs of modality and signs of quantity, various explanations may be offered. It may be, for example, that signs of modality are just ordinary quantifiers operating upon a peculiar subject-matter, namely possible states of affairs … It would not be quite accurate to describe theories of this sort as ‘reducing modality to quantity’. They do reduce modal distinctions to distinctions of quantity, but the variables to which the quantifiers are attached retain something modal in their signification — they signify ‘possibilities’, ‘chances’, ‘possible states of affairs’, ‘possible combinations of truth-values’, or the like. (1951: 736–7.)