Let P be a sentence which is true or false, but not both true and
false. The sentence ``P and Not(P)'' is known as a contradiction.
**Regardless of whether P is true, ``P and Not(P)'' is always
false.** If P is true, Not(P) is false and the ``and'' of the two
of them is false. If P is false, the ``and'' of the two of them is false.
The table below summarizes these facts:

P | Not(P) | P and Not(P) |
---|---|---|

T | F | F |

F | T | F |

Second, consider any sentences, P and Q, each of which is true or false and neither of which is both true and false. Consider the sentence, ``(P and Not(P)) or Q''. This means exactly the same as Q, because ``P and Not(P))'' is always false. This principle is more formally explained by the truth table below: note that columns 2 and 5 have the same truth values.

P | Q | Not(P) | P and Not(P) | [P and Not(P)] or Q |
---|---|---|---|---|

T | T | F | F | T |

T | F | F | F | F |

F | T | T | F | T |

F | F | T | F | F |

Third, let us continue with P and Q as above. The sentence ``if [P and
Not(P)], then Q'' is always true, regardless of the truth values of P and
Q. This is the principle that, **from a contradiction, anything (and
everything) follows as a logical conclusion.** The table below
explores the four possible cases, but the truth is simpler than that. In
an ``if---then'' sentence, if the sentence in the ``if'' part is false,
the entire ``if---then'' sentence is true. Since ``P and Not(P)'' is
always false, making it the ``if'' part of an ''if---then'' sentence
always produces a true sentence. This is like making a promise based on
conditions that you know will never be satisfied.

P | Q | Not(P) | P and Not(P) | If [P and Not(P)], then Q |
---|---|---|---|---|

T | T | F | F | T |

T | F | F | F | T |

F | T | T | F | T |

F | F | T | F | T |

Fourth, continue with P and Q as above. The sentence ``if Q, then [P
and Not(P)]'' means the same as ``Not(Q)''. This captures the principle of
**proof by contradiction. If some assumption such as ``Q'' implies a
contradiction such as ``P and Not(P)'', then ``Q'' is false. **Here
the ``then'' part is always false; the only way for the entire
``if---then'' to be true is that ``Q'' is likewise false (see
conditional). The truth table below presents this more formally: note
that columns 5 and 6 have the same truth values.

P | Q | Not(P) | P and Not(P) | If Q, then [P and Not(P)] | Not(Q) |
---|---|---|---|---|---|

T | T | F | F | F | F |

T | F | F | F | T | T |

F | T | T | F | F | F |

F | F | T | F | T | T |

Fifth, let us continue as above but play this time with ``{[Not(P)] or P} and Q''. Note that ``[Not(P) or P]'' is always true, and thus the truth of the entire ``and'' sentence is determined by Q. This sentence means the same as Q. Here is a truth table for this principle: note that columns 2 and 5 have the same truth values.

P | Q | Not(P) | [Not(P)] or P | {[Not(P)] or P} and Q |
---|---|---|---|---|

T | T | F | T | T |

T | F | F | T | F |

F | T | T | T | T |

F | F | T | T | F |

Sixth, with P and Q as above, consider ``If {[Not(P)] or P}, then Q''. Note that the ``if'' part is always true. So the truth of the whole ``if---then'' depends only upon Q; if Q is false the promise is broken and if Q is true the promise is kept. This sentence means the same as Q, as the following truth table formalizes: note that columns 2 and 5 have the same truth values.

P | Q | Not(P) | [Not(P)] or P | If {[Not(P)] or P}, then Q |
---|---|---|---|---|

T | T | F | T | T |

T | F | F | T | F |

F | T | T | T | T |

F | F | T | T | F |

Seventh, with P and Q as above, consider ``if Q, then {[Not(P)] or P}''. This time, the ``then'' part is always true which makes the entire ``if---then'' always true. It's like promising something that the universe always provides no matter what---it's any easy promise to keep! Here is a truth table for it:

P | Q | Not(P) | [Not(P)] or P | If Q, then {[Not(P)] or P} |
---|---|---|---|---|

T | T | F | T | T |

T | F | F | T | T |

F | T | T | T | T |

F | F | T | T | T |