contraries any pair of propositions that cannot both be true but can both be false; derivatively, any pair of properties that cannot both apply to a thing but that can both fail to apply to a thing. Thus the propositions ‘This object is red all over’ and ‘This object is green all over’ are contraries, as are the properties of being red all over and being green all over. Traditionally, it was considered that the categorical A-proposition ‘All S’s are P’s’ and the categorical E-proposition ‘No S’s are P’s’ were contraries; but according to De Morgan and most subsequent logicians, these two propositions are both true when there are no S’s at all, so that modern logicians do not usually regard the categorical A- and E-propositions as being true contraries. See also EXISTENTIAL IMPORT, SQUARE OF OPPOSITION , SYLLOGIS. R.W.B.