| 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 axiom is logically redundant in the (logically complete)
predicate calculus axiom system consisting of ax-gen 999, ax-4 1009 through
ax-9 1001, ax-10o 1177, and ax-12 1004 through ax-16 1247: in that system, we
can derive any instance of ax-17 1007 not containing wff variables by
induction on formula length, using ax17eq 1248 and ax17el 1400 for the basis
together hbn 1040, hbal 1041, and hbim 1043.
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). |