# Clavius's Law/Formulation 2/Proof 2

## Theorem

- $\vdash \left({\neg p \implies p}\right) \implies p$

## Proof

By the tableau method of natural deduction:

Line | Pool | Formula | Rule | Depends upon | Notes | |
---|---|---|---|---|---|---|

1 | 1 | $\neg p \implies p$ | Premise | (None) | ||

2 | $p \lor \neg p$ | Law of Excluded Middle | (None) | |||

3 | 3 | $\neg p$ | Assumption | (None) | Either $p$ is false ... | |

4 | 1, 3 | $p$ | Modus Ponendo Ponens: $\implies \mathcal E$ | 1, 3 | ||

5 | 5 | $p$ | Assumption | (None) | ... or $p$ is true | |

6 | 1 | $p$ | Proof by Cases: $\text{PBC}$ | 2, 3 – 4, 5 – 5 | Assumptions 3 and 5 have been discharged | |

7 | $\left({\neg p \implies p}\right) \implies p$ | Rule of Implication: $\implies \mathcal I$ | 1 – 6 | Assumption 1 has been discharged |

$\blacksquare$

## Law of the Excluded Middle

This theorem depends on the Law of the Excluded Middle.

This is one of the axioms of logic that was determined by Aristotle, and forms part of the backbone of classical (Aristotelian) logic.

However, the intuitionist school rejects the Law of the Excluded Middle as a valid logical axiom. This in turn invalidates this theorem from an intuitionistic perspective.

## Source of Name

This entry was named for Christopher Clavius.

## Sources

- 1964: Donald Kalish and Richard Montague:
*Logic: Techniques of Formal Reasoning*... (previous) ... (next): $\text{I}$: 'NOT' and 'IF': $\S 5$: Theorem $\text{T19}$