Theorem dcfromcon 1471

Description: The decidability of a proposition 𝜒 follows from a suitable instance of the principle of contraposition. Therefore, if we were to introduce contraposition as a general principle (without the decidability condition in condc 857), then we could prove that every proposition is decidable, giving us the classical system of propositional calculus (since the principle of contraposition is itself classically valid). (Contributed by Adrian Ducourtial, 6-Oct-2025.)

Hypotheses

Ref	Expression
dcfromcon.1	⊢ (𝜑 ↔ (𝜒 ∨ ¬ 𝜒))
dcfromcon.2	⊢ (𝜓 ↔ ⊤)
dcfromcon.3	⊢ ((¬ 𝜑 → ¬ 𝜓) → (𝜓 → 𝜑))

Assertion

Ref	Expression
dcfromcon	⊢ DECID 𝜒

Proof of Theorem dcfromcon

Step	Hyp	Ref	Expression
1		nnexmid 854	. . . . 5 ⊢ ¬ ¬ (𝜒 ∨ ¬ 𝜒)
2	1	pm2.21i 649	. . . 4 ⊢ (¬ (𝜒 ∨ ¬ 𝜒) → ¬ ⊤)
3		dcfromcon.3	. . . . 5 ⊢ ((¬ 𝜑 → ¬ 𝜓) → (𝜓 → 𝜑))
4		dcfromcon.1	. . . . . . 7 ⊢ (𝜑 ↔ (𝜒 ∨ ¬ 𝜒))
5	4	notbii 672	. . . . . 6 ⊢ (¬ 𝜑 ↔ ¬ (𝜒 ∨ ¬ 𝜒))
6		dcfromcon.2	. . . . . . 7 ⊢ (𝜓 ↔ ⊤)
7	6	notbii 672	. . . . . 6 ⊢ (¬ 𝜓 ↔ ¬ ⊤)
8	5, 7	imbi12i 239	. . . . 5 ⊢ ((¬ 𝜑 → ¬ 𝜓) ↔ (¬ (𝜒 ∨ ¬ 𝜒) → ¬ ⊤))
9	6, 4	imbi12i 239	. . . . 5 ⊢ ((𝜓 → 𝜑) ↔ (⊤ → (𝜒 ∨ ¬ 𝜒)))
10	3, 8, 9	3imtr3i 200	. . . 4 ⊢ ((¬ (𝜒 ∨ ¬ 𝜒) → ¬ ⊤) → (⊤ → (𝜒 ∨ ¬ 𝜒)))
11	2, 10	ax-mp 5	. . 3 ⊢ (⊤ → (𝜒 ∨ ¬ 𝜒))
12	11	mptru 1384	. 2 ⊢ (𝜒 ∨ ¬ 𝜒)
13		df-dc 839	. 2 ⊢ (DECID 𝜒 ↔ (𝜒 ∨ ¬ 𝜒))
14	12, 13	mpbir 146	1 ⊢ DECID 𝜒

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 105   ∨ wo 712  DECID wdc 838  ⊤wtru 1376

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-in1 617  ax-in2 618  ax-io 713

This theorem depends on definitions:  df-bi 117  df-dc 839  df-tru 1378

This theorem is referenced by: (None)