law of the excluded middle

Contradiction \Con`tra*dic"tion\, n. [L. contradictio answer,
   objection: cf. F. contradiction.]
   1. An assertion of the contrary to what has been said or
      affirmed; denial of the truth of a statement or assertion;
      contrary declaration; gainsaying.
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            His fair demands
            Shall be accomplished without contradiction. --Shak.
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   2. Direct opposition or repugnancy; inconsistency;
      incongruity or contrariety; one who, or that which, is
      inconsistent.
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            can he make deathless death? That were to make
            Strange contradiction.                --Milton.
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            We state our experience and then we come to a manly
            resolution of acting in contradiction to it.
                                                  --Burke.
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            Both parts of a contradiction can not possibly be
            true.                                 --Hobbes.
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            Of contradictions infinite the slave. --Wordsworth.
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   {Principle of contradiction} (Logic), the axiom or law of
      thought that a thing cannot be and not be at the same
      time, or a thing must either be or not be, or the same
      attribute can not at the same time be affirmed and and
      denied of the same subject; also called the {law of the
      excluded middle}.

   Note: It develops itself in three specific forms which have
         been called the "Three Logical Axioms." First, "A is
         A." Second, "A is not Not-A" Third, "Everything is
         either A or Not-A."
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