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Mirrors > Home > NFE Home > Th. List > peirce | GIF version |
Description: Peirce's axiom. This odd-looking theorem is the "difference" between an intuitionistic system of propositional calculus and a classical system and is not accepted by intuitionists. When Peirce's axiom is added to an intuitionistic system, the system becomes equivalent to our classical system ax-1 5 through ax-3 7. A curious fact about this theorem is that it requires ax-3 7 for its proof even though the result has no negation connectives in it. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 9-Oct-2012.) |
Ref | Expression |
---|---|
peirce | ⊢ (((φ → ψ) → φ) → φ) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | simplim 143 | . 2 ⊢ (¬ (φ → ψ) → φ) | |
2 | id 19 | . 2 ⊢ (φ → φ) | |
3 | 1, 2 | ja 153 | 1 ⊢ (((φ → ψ) → φ) → φ) |
Colors of variables: wff setvar class |
Syntax hints: → wi 4 |
This theorem was proved from axioms: ax-1 5 ax-2 6 ax-3 7 ax-mp 8 |
This theorem is referenced by: looinv 174 tbw-ax3 1467 exmoeu 2246 |
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