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Axiom ax-6 1685
Description: Axiom of Existence. One of the equality and substitution axioms of predicate calculus with equality. This axiom tells us is that at least one thing exists. In this form (not requiring that and be distinct) it was used in an axiom system of Tarski (see Axiom B7' in footnote 1 of [KalishMontague] p. 81.) It is equivalent to axiom scheme C10' in [Megill] p. 448 (p. 16 of the preprint); the equivalence is established by axc10 1961 and ax6fromc10 2258. A more convenient form of this axiom is a6e 1959, which has additional remarks.

Raph Levien proved the independence of this axiom from the other logical axioms on 12-Apr-2005. See item 16 at

ax-6 1685 can be proved from the weaker version ax6v 1686 requiring that the variables be distinct; see theorem ax6 1960.

ax-6 1685 can also be proved from the Axiom of Separation (in the form that we use that axiom, where free variables are not universally quantified). See theorem ax6vsep 4443.

Except by ax6v 1686, this axiom should not be referenced directly. Instead, use theorem ax6 1960. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.)

Ref Expression

Detailed syntax breakdown of Axiom ax-6
StepHypRef Expression
1 vx . . . . 5
2 vy . . . . 5
31, 2weq 1671 . . . 4
43wn 3 . . 3
54, 1wal 1564 . 2
65wn 3 1
Colors of variables: wff set class
This axiom is referenced by:  ax6v  1686
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