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|Description: Axiom of Quantifier
Introduction. One of the equality and substitution
axioms of predicate calculus with equality.
An equivalent way to express this axiom that may be easier to understand is . Recall that in the intended interpretation, our variables are metavariables ranging over the variables of predicate calculus (the object language). In order for the first antecedent to hold, and must have different values and thus cannot be the same object-language variable. Similarly, and cannot be the same object-language variable. Therefore, will not occur in the wff when the first two antecedents hold, so analogous to ax-17 1628, the conclusion follows.
To simplify the above form of the axiom, we exploit the antecedent to show that is equivalent to , so one of them is redundant and can be discarded as ax12b 1834 shows.
The original version of this axiom was ax-12o 1664 ("o" for "old") and was replaced with this shorter ax-12 1633 in December 2015. The old axiom is proved from this one as theorem ax12o 1663. Conversely, this axiom is proved from ax-12o 1664 as theorem ax12 1882.
Although this version is shorter, the original version ax-12o 1664 may be more practical to work with because of the "distinctor" form of its antecedents.
This axiom can be weakened if desired by adding distinct variable restrictions on pairs and . To show that, we add these restrictions to theorem ax12v 1634 and use only ax12v 1634 for further derivations. Thus ax12v 1634 should be the only theorem referencing this axiom. Other theorems can reference either ax12v 1634 or ax-12o 1664. (Contributed by NM, 21-Dec-2015.) (New usage is discouraged.)
|1||vx||. . . 4|
|2||vy||. . . 4|
|3||1, 2||weq 1620||. . 3|
|4||3||wn 5||. 2|
|5||vz||. . . 4|
|6||2, 5||weq 1620||. . 3|
|7||6, 1||wal 1532||. . 3|
|8||6, 7||wi 6||. 2|
|9||4, 8||wi 6||1|
|Colors of variables: wff set class|
|This axiom is referenced by: ax12v 1634 ax12vX 28281|
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