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Necessary or necessity may refer to:

  • Need
    • An action somebody may feel they must do
    • An important task or essential thing to do at a particular time or by a particular moment
  • Necessary and sufficient condition, in logic, something that is a required condition for something else to be the case
  • Necessary proposition, in logic, a statement about facts that is either unassailably true (tautology) or obviously false (contradiction)
  • Metaphysical necessity, in philosophy, a truth which is true in all possible worlds
  • Necessity in modal logic

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