| 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 1305, ax-4 1319 through
ax-9 1307, ax-10o 1500, and ax-12 1310 through ax-16 1580: in that system, we can
derive any instance of ax-17 1317 not containing wff variables by induction
on formula length, using ax17eq 1581 and ax17el 1752 for the basis together
hbn 1351, hbal 1352, and hbim 1354.
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). |