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 axc101961 and ax6fromc102258. A more convenient form of this
axiom is a6e1959, 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
http://us.metamath.org/award2003.html.

ax-61685 can be proved from the weaker version ax6v1686
requiring that the
variables be distinct; see theorem ax61960.

ax-61685 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 ax6vsep4443.

Except by ax6v1686, this axiom should not be referenced
directly. Instead,
use theorem ax61960. (Contributed by NM, 5-Aug-1993.)
(New usage is discouraged.)