From Late Latin tautologia, from Ancient Greek ταυτολογία ‎(tautología) from ταὐτός ‎(tautós, the same) + λόγος ‎(lógos, explanation)



tautology ‎(countable and uncountable, plural tautologies)

  1. (uncountable) Redundant use of words.
    It is tautology to say, "Forward Planning".
  2. (countable) An expression that features tautology.
    The expression "raze to the ground" is a tautology, since the word "raze" includes the notion "to the ground".
    • 1946, Bertrand Russell, History of Western Philosophy:
      Pure mathematics consists of tautologies, analogous to ‘men are men’, but usually more complicated.
  3. (countable, logic) A statement that is true for all values of its variables.
    Given a Boolean A, "A OR (NOT A)" is a tautology.
    A logical statement which is neither a tautology nor a contradiction is a contingency.
    A tautology can be verified by constructing a truth tree for its negation: if all of the leaf nodes of such truth tree end in X's, then the original (pre-negated) formula is a tautology.


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