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. If we allow the
otherwise redundant ax-15 1109 and ax-16 1194, this axiom is logically
redundant since it is a metatheorem justified by induction on the number
of primitive connectives in wff , using ax17eq 1195 and ax17el 1196
together hbn 980, hbal 981, and hbim 983.
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
is effectively a definition of the concept of a set variable not
occurring in a wff (as opposed to just two set variables being
distinct). |