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Theorem pm4.81ALT 31580
Description: Alternate proof of pm4.81 379. (Contributed by BJ, 30-Mar-2020.) (Proof modification is discouraged.) (New usage is discouraged.)
Assertion
Ref Expression
pm4.81ALT ((¬ 𝜑𝜑) ↔ 𝜑)

Proof of Theorem pm4.81ALT
StepHypRef Expression
1 pm2.18 120 . 2 ((¬ 𝜑𝜑) → 𝜑)
2 ax-1 6 . 2 (𝜑 → (¬ 𝜑𝜑))
31, 2impbii 197 1 ((¬ 𝜑𝜑) ↔ 𝜑)
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4  wb 194
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8
This theorem depends on definitions:  df-bi 195
This theorem is referenced by: (None)
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