Theorem wl-dfcleq 37891

Description: The defining characterization of class equality. This version of df-cleq 2733 has no restrictions, unlike the forms on which it is based. It is proved in Tarski's FOL from the axiom of extensionality (ax-ext 2713), the definition of class equality (df-cleq 2733), and the definition of class membership (df-clel 2816).

Its forward implication is known as "class extensionality". (Contributed by NM, 15-Sep-1993.) (Revised by BJ, 24-Jun-2019.) Base on wl-dfcleq.just 37887. (Revised by Wolf Lammen, 7-Apr-2026.)

Assertion

Ref	Expression
wl-dfcleq	⊢ (𝐴 = 𝐵 ↔ ∀𝑥(𝑥 ∈ 𝐴 ↔ 𝑥 ∈ 𝐵))

Distinct variable groups:   𝑥,𝐴   𝑥,𝐵

Proof of Theorem wl-dfcleq

Dummy variables 𝑦 𝐶 are mutually distinct and distinct from all other variables.

Step	Hyp	Ref	Expression
1		eleq1w 2824	. . . 4 ⊢ (𝑥 = 𝑦 → (𝑥 ∈ 𝐴 ↔ 𝑦 ∈ 𝐴))
2		eleq1w 2824	. . . 4 ⊢ (𝑥 = 𝑦 → (𝑥 ∈ 𝐵 ↔ 𝑦 ∈ 𝐵))
3	1, 2	bibi12d 347	. . 3 ⊢ (𝑥 = 𝑦 → ((𝑥 ∈ 𝐴 ↔ 𝑥 ∈ 𝐵) ↔ (𝑦 ∈ 𝐴 ↔ 𝑦 ∈ 𝐵)))
4	3	cbvalvw 2044	. 2 ⊢ (∀𝑥(𝑥 ∈ 𝐴 ↔ 𝑥 ∈ 𝐵) ↔ ∀𝑦(𝑦 ∈ 𝐴 ↔ 𝑦 ∈ 𝐵))
5		eqid 2741	. 2 ⊢ 𝐴 = 𝐴
6		eqtr 2761	. . . 4 ⊢ ((𝐴 = 𝐵 ∧ 𝐵 = 𝐶) → 𝐴 = 𝐶)
7	6	eqcomd 2747	. . 3 ⊢ ((𝐴 = 𝐵 ∧ 𝐵 = 𝐶) → 𝐶 = 𝐴)
8	7	ex 414	. 2 ⊢ (𝐴 = 𝐵 → (𝐵 = 𝐶 → 𝐶 = 𝐴))
9		eqeq2 2753	. . . . . 6 ⊢ (𝐴 = 𝐵 → (𝑥 = 𝐴 ↔ 𝑥 = 𝐵))
10	9	biimpd 231	. . . . 5 ⊢ (𝐴 = 𝐵 → (𝑥 = 𝐴 → 𝑥 = 𝐵))
11	10	anim1d 618	. . . 4 ⊢ (𝐴 = 𝐵 → ((𝑥 = 𝐴 ∧ 𝑥 ∈ 𝐶) → (𝑥 = 𝐵 ∧ 𝑥 ∈ 𝐶)))
12	11	eximdv 1925	. . 3 ⊢ (𝐴 = 𝐵 → (∃𝑥(𝑥 = 𝐴 ∧ 𝑥 ∈ 𝐶) → ∃𝑥(𝑥 = 𝐵 ∧ 𝑥 ∈ 𝐶)))
13		dfclel 2817	. . 3 ⊢ (𝐴 ∈ 𝐶 ↔ ∃𝑥(𝑥 = 𝐴 ∧ 𝑥 ∈ 𝐶))
14		dfclel 2817	. . 3 ⊢ (𝐵 ∈ 𝐶 ↔ ∃𝑥(𝑥 = 𝐵 ∧ 𝑥 ∈ 𝐶))
15	12, 13, 14	3imtr4g 298	. 2 ⊢ (𝐴 = 𝐵 → (𝐴 ∈ 𝐶 → 𝐵 ∈ 𝐶))
16		wl-dfcleq.basic 37886	. . . . . 6 ⊢ (𝐴 = 𝐵 ↔ ∀𝑦(𝑦 ∈ 𝐴 ↔ 𝑦 ∈ 𝐵))
17		biimp 217	. . . . . . . 8 ⊢ ((𝑦 ∈ 𝐴 ↔ 𝑦 ∈ 𝐵) → (𝑦 ∈ 𝐴 → 𝑦 ∈ 𝐵))
18	17	alimi 1819	. . . . . . 7 ⊢ (∀𝑦(𝑦 ∈ 𝐴 ↔ 𝑦 ∈ 𝐵) → ∀𝑦(𝑦 ∈ 𝐴 → 𝑦 ∈ 𝐵))
19	1	biimpd 231	. . . . . . . . . 10 ⊢ (𝑥 = 𝑦 → (𝑥 ∈ 𝐴 → 𝑦 ∈ 𝐴))
20	19	eqcoms 2749	. . . . . . . . 9 ⊢ (𝑦 = 𝑥 → (𝑥 ∈ 𝐴 → 𝑦 ∈ 𝐴))
21		eleq1w 2824	. . . . . . . . . 10 ⊢ (𝑦 = 𝑥 → (𝑦 ∈ 𝐵 ↔ 𝑥 ∈ 𝐵))
22	21	biimpd 231	. . . . . . . . 9 ⊢ (𝑦 = 𝑥 → (𝑦 ∈ 𝐵 → 𝑥 ∈ 𝐵))
23	20, 22	imim12d 81	. . . . . . . 8 ⊢ (𝑦 = 𝑥 → ((𝑦 ∈ 𝐴 → 𝑦 ∈ 𝐵) → (𝑥 ∈ 𝐴 → 𝑥 ∈ 𝐵)))
24	23	spimvw 1994	. . . . . . 7 ⊢ (∀𝑦(𝑦 ∈ 𝐴 → 𝑦 ∈ 𝐵) → (𝑥 ∈ 𝐴 → 𝑥 ∈ 𝐵))
25	18, 24	syl 17	. . . . . 6 ⊢ (∀𝑦(𝑦 ∈ 𝐴 ↔ 𝑦 ∈ 𝐵) → (𝑥 ∈ 𝐴 → 𝑥 ∈ 𝐵))
26	16, 25	sylbi 219	. . . . 5 ⊢ (𝐴 = 𝐵 → (𝑥 ∈ 𝐴 → 𝑥 ∈ 𝐵))
27	26	anim2d 619	. . . 4 ⊢ (𝐴 = 𝐵 → ((𝑥 = 𝐶 ∧ 𝑥 ∈ 𝐴) → (𝑥 = 𝐶 ∧ 𝑥 ∈ 𝐵)))
28	27	eximdv 1925	. . 3 ⊢ (𝐴 = 𝐵 → (∃𝑥(𝑥 = 𝐶 ∧ 𝑥 ∈ 𝐴) → ∃𝑥(𝑥 = 𝐶 ∧ 𝑥 ∈ 𝐵)))
29		dfclel 2817	. . 3 ⊢ (𝐶 ∈ 𝐴 ↔ ∃𝑥(𝑥 = 𝐶 ∧ 𝑥 ∈ 𝐴))
30		dfclel 2817	. . 3 ⊢ (𝐶 ∈ 𝐵 ↔ ∃𝑥(𝑥 = 𝐶 ∧ 𝑥 ∈ 𝐵))
31	28, 29, 30	3imtr4g 298	. 2 ⊢ (𝐴 = 𝐵 → (𝐶 ∈ 𝐴 → 𝐶 ∈ 𝐵))
32	4, 5, 8, 15, 31	wl-dfcleq.just 37887	1 ⊢ (𝐴 = 𝐵 ↔ ∀𝑥(𝑥 ∈ 𝐴 ↔ 𝑥 ∈ 𝐵))

Syntax hints:   → wi 4   ↔ wb 208   ∧ wa 397  ∀wal 1546   = wceq 1548  ∃wex 1787   ∈ wcel 2121

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1803  ax-4 1817  ax-5 1918  ax-6 1975  ax-7 2016  ax-8 2123  ax-9 2131  ax-ext 2713

This theorem depends on definitions:  df-bi 209  df-an 398  df-ex 1788  df-cleq 2733  df-clel 2816

This theorem is referenced by: (None)