Theorem isset 2777

Description: Two ways to say "𝐴 is a set": A class 𝐴 is a member of the universal class V (see df-v 2773) if and only if the class 𝐴 exists (i.e. there exists some set 𝑥 equal to class 𝐴). Theorem 6.9 of [Quine] p. 43. Notational convention: We will use the notational device "𝐴 ∈ V " to mean "𝐴 is a set" very frequently, for example in uniex 4482. Note the when 𝐴 is not a set, it is called a proper class. In some theorems, such as uniexg 4484, in order to shorten certain proofs we use the more general antecedent 𝐴 ∈ 𝑉 instead of 𝐴 ∈ V to mean "𝐴 is a set."

Note that a constant is implicitly considered distinct from all variables. This is why V is not included in the distinct variable list, even though df-clel 2200 requires that the expression substituted for 𝐵 not contain 𝑥. (Also, the Metamath spec does not allow constants in the distinct variable list.) (Contributed by NM, 26-May-1993.)

Assertion

Ref	Expression
isset	⊢ (𝐴 ∈ V ↔ ∃𝑥 𝑥 = 𝐴)

Distinct variable group:   𝑥,𝐴

Proof of Theorem isset

Step	Hyp	Ref	Expression
1		df-clel 2200	. 2 ⊢ (𝐴 ∈ V ↔ ∃𝑥(𝑥 = 𝐴 ∧ 𝑥 ∈ V))
2		vex 2774	. . . 4 ⊢ 𝑥 ∈ V
3	2	biantru 302	. . 3 ⊢ (𝑥 = 𝐴 ↔ (𝑥 = 𝐴 ∧ 𝑥 ∈ V))
4	3	exbii 1627	. 2 ⊢ (∃𝑥 𝑥 = 𝐴 ↔ ∃𝑥(𝑥 = 𝐴 ∧ 𝑥 ∈ V))
5	1, 4	bitr4i 187	1 ⊢ (𝐴 ∈ V ↔ ∃𝑥 𝑥 = 𝐴)

Syntax hints:   ∧ wa 104   ↔ wb 105   = wceq 1372  ∃wex 1514   ∈ wcel 2175  Vcvv 2771

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-5 1469  ax-gen 1471  ax-ie1 1515  ax-ie2 1516  ax-8 1526  ax-4 1532  ax-17 1548  ax-i9 1552  ax-ial 1556  ax-ext 2186

This theorem depends on definitions:  df-bi 117  df-sb 1785  df-clab 2191  df-cleq 2197  df-clel 2200  df-v 2773

This theorem is referenced by:  issetf  2778  isseti  2779  issetri  2780  elex  2782  elisset  2785  vtoclg1f  2831  ceqex  2899  eueq  2943  moeq  2947  mosubt  2949  ru  2996  sbc5  3021  snprc  3697  snssb  3765  vprc  4175  opelopabsb  4304  eusvnfb  4499  elrelimasn  5045  euiotaex  5245  fvmptdf  5661  fvmptdv2  5663  fmptco  5740  brabvv  5981  ovmpodf  6067  ovi3  6073  tfrlemibxssdm  6403  tfr1onlembxssdm  6419  tfrcllembxssdm  6432  ecexr  6615  snexxph  7034  fnpr2ob  13090  bj-vprc  15696  bj-vnex  15698  bj-2inf  15738