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Theorem condc 749
Description: Contraposition of a decidable proposition.

This theorem swaps or "transposes" the order of the consequents when negation is removed. An informal example is that the statement "if there are no clouds in the sky, it is not raining" implies the statement "if it is raining, there are clouds in the sky." This theorem (without the decidability condition, of course) is called Transp or "the principle of transposition" in Principia Mathematica (Theorem *2.17 of [WhiteheadRussell] p. 103) and is Axiom A3 of [Margaris] p. 49. We will also use the term "contraposition" for this principle, although the reader is advised that in the field of philosophical logic, "contraposition" has a different technical meaning.

(Contributed by Jim Kingdon, 13-Mar-2018.)

Assertion
Ref Expression
condc (DECID 𝜑 → ((¬ 𝜑 → ¬ 𝜓) → (𝜓𝜑)))

Proof of Theorem condc
StepHypRef Expression
1 df-dc 743 . 2 (DECID 𝜑 ↔ (𝜑 ∨ ¬ 𝜑))
2 ax-1 5 . . . 4 (𝜑 → (𝜓𝜑))
32a1d 22 . . 3 (𝜑 → ((¬ 𝜑 → ¬ 𝜓) → (𝜓𝜑)))
4 pm2.27 35 . . . 4 𝜑 → ((¬ 𝜑 → ¬ 𝜓) → ¬ 𝜓))
5 ax-in2 545 . . . 4 𝜓 → (𝜓𝜑))
64, 5syl6 29 . . 3 𝜑 → ((¬ 𝜑 → ¬ 𝜓) → (𝜓𝜑)))
73, 6jaoi 636 . 2 ((𝜑 ∨ ¬ 𝜑) → ((¬ 𝜑 → ¬ 𝜓) → (𝜓𝜑)))
81, 7sylbi 114 1 (DECID 𝜑 → ((¬ 𝜑 → ¬ 𝜓) → (𝜓𝜑)))
Colors of variables: wff set class
Syntax hints:  ¬ wn 3  wi 4  wo 629  DECID wdc 742
This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  ax-ia1 99  ax-ia2 100  ax-ia3 101  ax-in2 545  ax-io 630
This theorem depends on definitions:  df-bi 110  df-dc 743
This theorem is referenced by:  pm2.18dc  750  con1dc  753  con4biddc  754  pm2.521dc  764  con34bdc  765  necon4aidc  2270  necon4addc  2272  necon4bddc  2273  necon4ddc  2274  nn0n0n1ge2b  8254
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