| 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. If we allow the
otherwise redundant ax-15 1353 and
ax-16 1206, this axiom is logically redundant since it
is a metatheorem
justified by induction on the number of primitive connectives in wff
φ, using ax17eq 1207 and ax17el 1354 together hbn 1001,
hbal 1002, and
hbim 1004. 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). |