7786 | Propositional logic handles negation, disjunction, conjunction; predicate logic adds quantifiers, predicates, relations |

7798 | There are three axiom schemas for propositional logic |

7799 | Proposition logic has definitions for its three operators: or, and, and identical |

7797 | Axiom systems of logic contain axioms, inference rules, and definitions of proof and theorems |

7794 | There are seven modalities in S4, each with its negation |

7791 | The simplest of the logics based on possible worlds is Lewis's S5 |

7793 | ◊p → □◊p is the hallmark of S5 |

7795 | S5 has just six modalities, and all strings can be reduced to those |

7787 | Possible worlds logics use true-in-a-world rather than true |

7788 | Modal logic has four basic modal negation equivalences |

7796 | Modal logics were studied in terms of axioms, but now possible worlds semantics is added |

7789 | Necessary implication is called 'strict implication'; if successful, it is called 'entailment' |

7790 | If an argument is invalid, a truth tree will indicate a counter-example |

7800 | Analytic truths are divided into logically and conceptually necessary |

7801 | Possibilities can be logical, theoretical, physical, economic or human |

7792 | A world has 'access' to a world it generates, which is important in possible worlds semantics |