\Con"tra*ries\ (? or ?; 48), n. pl. [Pl. of
{Contrary}, n.] (Logic)
Propositions which directly and destructively contradict each
other, but of which the falsehood of one does not establish
the truth of the other.

      If two universals differ in quality, they are
      contraries; as, every vine is a tree; no vine is a
      tree. These can never be both true together; but they
      may be both false.                       --I. Watts.