Propositional form X is the subcontrary of propositional form Y (i.e., X and Y are subcontraries [of one another]) iff (1) it is possible for both X and Y to be true and (2) it is impossible for both X and Y to be false.
Example from everyday life: 'There is a page in this book containing misprints' is the subcontrary of 'There is a page in this book containing no misprints' (from Cohen and Nagel)
Example from categorical logic: 'Some S are P' is the subcontrary of 'Some S are not P' (iff at least one S exists)
Example from propositional logic: 'p ∨ q' is the subcontrary of '~p ∨ ~q'
Example from predicate logic: '(Ǝx)Øx' is the subcontrary of '(Ǝx)~Øx' (iff at least one object exists)
