{
  "aliases": [
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  "body": "\n## Summary\n\nIn logic and philosophy, **S5** is one of five systems of modal logic proposed by Clarence Irving Lewis and Cooper Harold Langford in their 1932 book *Symbolic Logic*. It is a normal modal logic, and one of the oldest systems of modal logic of any kind. It is formed with propositional calculus formulas and [[tautologies|05006AJ7DZ]], and inference apparatus with substitution and [[modus ponens|0Y0MQYST0M]], but extending the syntax with the modal operator *necessarily* $\\Box$ and its dual *possibly* $\\Diamond$.\n",
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  "name": "S5 (modal logic)",
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