|Description: Axiom of Substitution.
One of the 5 equality axioms of predicate
calculus. The final consequent ∀𝑥(𝑥 = 𝑦 → 𝜑) is a way of
expressing "𝑦 substituted for 𝑥 in wff
" (cf. sb6 2309). It
is based on Lemma 16 of [Tarski] p. 70 and
Axiom C8 of [Monk2] p. 105,
from which it can be proved by cases.
The original version of this axiom was ax-c15 34964 and was replaced with this
shorter ax-12 2222 in Jan. 2007. The old axiom is proved from
this one as
theorem axc15 2443. Conversely, this axiom is proved from ax-c15 34964 as
theorem ax12 2444.
Juha Arpiainen proved the metalogical independence of this axiom (in the
form of the older axiom ax-c15 34964) from the others on 19-Jan-2006.
item 9a at http://us.metamath.org/award2003.html.
See ax12v 2223 and ax12v2 2224 for other equivalents of this axiom that
this axiom) have distinct variable restrictions.
This axiom scheme is logically redundant (see ax12w 2186) but is used as an
auxiliary axiom scheme to achieve scheme completeness. (Contributed by
NM, 22-Jan-2007.) (New usage is discouraged.)