Theorem dcfrompeirce 1494

Description: The decidability of a proposition ch follows from a suitable instance of Peirce's law. Therefore, if we were to introduce Peirce's law as a general principle (without the decidability condition in peircedc 921), then we could prove that every proposition is decidable, giving us the classical system of propositional calculus (since Perice's law is itself classically valid). (Contributed by Adrian Ducourtial, 6-Oct-2025.)

Hypotheses

Ref	Expression
dcfrompeirce.1	|- ( ph <-> ( ch \/ -. ch ) )
dcfrompeirce.2	|- ( ps <-> F. )
dcfrompeirce.3	|- ( ( ( ph -> ps ) -> ph ) -> ph )

Assertion

Ref	Expression
dcfrompeirce	|- DECID ch

Proof of Theorem dcfrompeirce

Step	Hyp	Ref	Expression
1		pm2.67-2 720	. . . . 5 |- ( ( ( ch \/ -. ch ) -> F. ) -> ( ch -> F. ) )
2		dfnot 1415	. . . . 5 |- ( -. ch <-> ( ch -> F. ) )
3	1, 2	sylibr 134	. . . 4 |- ( ( ( ch \/ -. ch ) -> F. ) -> -. ch )
4	3	olcd 741	. . 3 |- ( ( ( ch \/ -. ch ) -> F. ) -> ( ch \/ -. ch ) )
5		dcfrompeirce.3	. . . 4 |- ( ( ( ph -> ps ) -> ph ) -> ph )
6		dcfrompeirce.1	. . . . . 6 |- ( ph <-> ( ch \/ -. ch ) )
7		dcfrompeirce.2	. . . . . 6 |- ( ps <-> F. )
8	6, 7	imbi12i 239	. . . . 5 |- ( ( ph -> ps ) <-> ( ( ch \/ -. ch ) -> F. ) )
9	8, 6	imbi12i 239	. . . 4 |- ( ( ( ph -> ps ) -> ph ) <-> ( ( ( ch \/ -. ch ) -> F. ) -> ( ch \/ -. ch ) ) )
10	5, 9, 6	3imtr3i 200	. . 3 |- ( ( ( ( ch \/ -. ch ) -> F. ) -> ( ch \/ -. ch ) ) -> ( ch \/ -. ch ) )
11	4, 10	ax-mp 5	. 2 |- ( ch \/ -. ch )
12		df-dc 842	. 2 |- (DECID ch <-> ( ch \/ -. ch ) )
13	11, 12	mpbir 146	1 |- DECID ch

Syntax hints:   -. wn 3   -> wi 4   <-> wb 105   \/ wo 715  DECID wdc 841   F. wfal 1402

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 619  ax-in2 620  ax-io 716

This theorem depends on definitions:  df-bi 117  df-dc 842  df-tru 1400  df-fal 1403

This theorem is referenced by: (None)