| 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 965, ax-4 975 through
ax-9 967, ax-10o 1142, and ax-12 970
through ax-16 1212: in that system, we
can derive any instance of ax-17 973 not containing wff variables by
induction on formula length, using ax17eq 1213 and ax17el 1363 for the basis
together hbn 1006, hbal 1007, and hbim 1009.
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). |