Theorem notnot 630

Description: Double negation introduction. Theorem *2.12 of [WhiteheadRussell] p. 101. The converse need not hold. It holds exactly for stable propositions (by definition, see df-stab 832) and in particular for decidable propositions (see notnotrdc 844). See also notnotnot 635. (Contributed by NM, 28-Dec-1992.) (Proof shortened by Wolf Lammen, 2-Mar-2013.)

Assertion

Ref	Expression
notnot	⊢ (𝜑 → ¬ ¬ 𝜑)

Proof of Theorem notnot

Step	Hyp	Ref	Expression
1		id 19	. 2 ⊢ (¬ 𝜑 → ¬ 𝜑)
2	1	con2i 628	1 ⊢ (𝜑 → ¬ ¬ 𝜑)

Syntax hints:  ¬ wn 3   → wi 4

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-in1 615  ax-in2 616

This theorem is referenced by:  notnotd  631  con3d  632  notnotnot  635  notnoti  646  pm3.24  694  biortn  746  dcn  843  con1dc  857  notnotbdc  873  imanst  889  eueq2dc  2945  ddifstab  3304  ifnotdc  3608  ismkvnex  7256  xrlttri3  9918  nltpnft  9935  ngtmnft  9938  bj-nnsn  15602  bj-nndcALT  15627  bdnthALT  15704