Theorem dcfromcon 1459

Description: The decidability of a proposition ch 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 854), 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	|- ( ph <-> ( ch \/ -. ch ) )
dcfromcon.2	|- ( ps <-> T. )
dcfromcon.3	|- ( ( -. ph -> -. ps ) -> ( ps -> ph ) )

Assertion

Ref	Expression
dcfromcon	|- DECID ch

Proof of Theorem dcfromcon

Step	Hyp	Ref	Expression
1		nnexmid 851	. . . . 5 |- -. -. ( ch \/ -. ch )
2	1	pm2.21i 647	. . . 4 |- ( -. ( ch \/ -. ch ) -> -. T. )
3		dcfromcon.3	. . . . 5 |- ( ( -. ph -> -. ps ) -> ( ps -> ph ) )
4		dcfromcon.1	. . . . . . 7 |- ( ph <-> ( ch \/ -. ch ) )
5	4	notbii 669	. . . . . 6 |- ( -. ph <-> -. ( ch \/ -. ch ) )
6		dcfromcon.2	. . . . . . 7 |- ( ps <-> T. )
7	6	notbii 669	. . . . . 6 |- ( -. ps <-> -. T. )
8	5, 7	imbi12i 239	. . . . 5 |- ( ( -. ph -> -. ps ) <-> ( -. ( ch \/ -. ch ) -> -. T. ) )
9	6, 4	imbi12i 239	. . . . 5 |- ( ( ps -> ph ) <-> ( T. -> ( ch \/ -. ch ) ) )
10	3, 8, 9	3imtr3i 200	. . . 4 |- ( ( -. ( ch \/ -. ch ) -> -. T. ) -> ( T. -> ( ch \/ -. ch ) ) )
11	2, 10	ax-mp 5	. . 3 |- ( T. -> ( ch \/ -. ch ) )
12	11	mptru 1373	. 2 |- ( ch \/ -. ch )
13		df-dc 836	. 2 |- (DECID ch <-> ( ch \/ -. ch ) )
14	12, 13	mpbir 146	1 |- DECID ch

Syntax hints:   -. wn 3   -> wi 4   <-> wb 105   \/ wo 709  DECID wdc 835   T. wtru 1365

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 615  ax-in2 616  ax-io 710

This theorem depends on definitions:  df-bi 117  df-dc 836  df-tru 1367

This theorem is referenced by: (None)