The Collaborative International Dictionary of English v.0.48:

Contraries \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.
   [1913 Webster]

         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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Contrary \Con"tra*ry\, n.; pl. Contraries.
   1. A thing that is of contrary or opposite qualities.
      [1913 Webster]

            No contraries hold more antipathy
            Than I and such a knave.              --Shak.
      [1913 Webster]

   2. An opponent; an enemy. [Obs.] --Chaucer.
      [1913 Webster]

   3. the opposite; a proposition, fact, or condition
      incompatible with another; as, slender proofs which rather
      show the contrary. See Converse, n., 1. --Locke.
      [1913 Webster]

   4. (Logic) See Contraries.
      [1913 Webster]

   On the contrary, in opposition; on the other hand. --Swift.

   To the contrary, to an opposite purpose or intent; on the
      other side. "They did it, not for want of instruction to
      the contrary." --Bp. Stillingfleet.
      [1913 Webster]