| 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 963, ax-4 973 through
ax-9 965, ax-10o 1140, and ax-12 968
through ax-16 1210: in that system, we
can derive any instance of ax-17 971 not containing wff variables by
induction on formula length, using ax17eq 1211 and ax17el 1361 for the basis
together hbn 1004, hbal 1005, and hbim 1007.
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). |