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Axiom ax-c15 32525
 Description: Axiom ax-c15 32525 was the original version of ax-12 1950, before it was discovered (in Jan. 2007) that the shorter ax-12 1950 could replace it. It appears as Axiom scheme C15' in [Megill] p. 448 (p. 16 of the preprint). 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. To understand this theorem more easily, think of " ..." as informally meaning "if and are distinct variables then..." The antecedent becomes false if the same variable is substituted for and , ensuring the theorem is sound whenever this is the case. In some later theorems, we call an antecedent of the form a "distinctor." Interestingly, if the wff expression substituted for contains no wff variables, the resulting statement can be proved without invoking this axiom. This means that even though this axiom is metalogically independent from the others, it is not logically independent. Specifically, we can prove any wff-variable-free instance of axiom ax-c15 32525 (from which the ax-12 1950 instance follows by theorem ax12 2190.) The proof is by induction on formula length, using ax12eq 32576 and ax12el 32577 for the basis steps and ax12indn 32578, ax12indi 32579, and ax12inda 32583 for the induction steps. (This paragraph is true provided we use ax-c11 32523 in place of ax-c11n 32524.) This axiom is obsolete and should no longer be used. It is proved above as theorem axc15 2153, which should be used instead. (Contributed by NM, 14-May-1993.) (New usage is discouraged.)
Assertion
Ref Expression
ax-c15

Detailed syntax breakdown of Axiom ax-c15
StepHypRef Expression
1 vx . . . . 5
2 vy . . . . 5
31, 2weq 1799 . . . 4
43, 1wal 1450 . . 3
54wn 3 . 2
6 wph . . . 4
73, 6wi 4 . . . . 5
87, 1wal 1450 . . . 4
96, 8wi 4 . . 3
103, 9wi 4 . 2
115, 10wi 4 1
 Colors of variables: wff setvar class This axiom is referenced by:  ax12o  32539
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