|Description: Axiom to quantify a
variable over a formula in which it does not occur.
Axiom C5 in [Megill] p. 444 (p. 11 of
the preprint). Also appears as
Axiom B6 (p. 75) of system S2 of [Tarski] p. 77 and Axiom C5-1 of
[Monk2] p. 113.
(This theorem simply repeats ax-17 1623 so that we can include the
following note, which applies only to the obsolete axiomatization.)
This axiom is logically redundant in the (logically complete)
predicate calculus axiom system consisting of ax-gen 1552, ax-5o 2170,
ax-4 2169, ax-7 1741, ax-6o 2171, ax-8 1682, ax-12o 2176, ax-9o 2172, ax-10o 2173,
ax-13 1719, ax-14 1721, ax-15 2177, ax-11o 2175, and ax-16 2178: in that system,
we can derive any instance of ax-17 1623 not containing wff variables by
induction on formula length, using ax17eq 2217 and ax17el 2223 for the basis
together hbn 1784, hbal 1743, and hbim 1826.
However, if we omit this axiom,
our development would be quite inconvenient since we could work only
with specific instances of wffs containing no wff variables - this axiom
introduces the concept of a set variable not occurring in a wff (as
opposed to just two set variables being distinct). (Contributed by NM,
19-Aug-2017.) (New usage is discouraged.) (Proof modification