MPE Home Metamath Proof Explorer < Previous   Next >
Nearby theorems
Mirrors  >  Home  >  MPE Home  >  Th. List  >  ax-12 Unicode version

Axiom ax-12 1868
Description: Axiom of Quantified Equality. 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  ( -.  x  =  y  ->  ( -.  x  =  z  -> 
( y  =  z  ->  A. x y  =  z ) ) ) (see ax12b 1657). 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  -.  x  =  y to hold,  x and  y must have different values and thus cannot be the same object-language variable. Similarly,  x and  z cannot be the same object-language variable. Therefore,  x will not occur in the wff  y  =  z when the first two antecedents hold, so analogous to ax-17 1605, the conclusion  ( y  =  z  ->  A. x
y  =  z ) follows.

The original version of this axiom was ax-12o 2083 and was replaced with this shorter ax-12 1868 in December 2015. The old axiom is proved from this one as theorem ax12o 1877. Conversely, this axiom is proved from ax-12o 2083 as theorem ax12 2097.

The primary purpose of this axiom is to provide a way to introduce the quantifier  A. x on  y  =  z even when  x and  y are substituted with the same variable. In this case, the first antecedent becomes  -.  x  =  x and the axiom still holds.

Although this version is shorter, the original version ax12o 1877 may be more practical to work with because of the "distinctor" form of its antecedents. A typical application of ax12o 1877 is in dvelimh 1906 which converts a distinct variable pair to the distinctor antecendent  -.  A. x x  =  y.

This axiom can be weakened if desired by adding distinct variable restrictions on pairs  x ,  z and  y ,  z. To show that, we add these restrictions to theorem ax12v 1869 and use only ax12v 1869 for further derivations. Thus, ax12v 1869 should be the only theorem referencing this axiom. Other theorems can reference either ax12v 1869 or ax12o 1877.

This axiom scheme is logically redundant (see ax12w 1700) but is used as an auxiliary axiom to achieve metalogical completeness. (Contributed by NM, 21-Dec-2015.) (New usage is discouraged.)

Assertion
Ref Expression
ax-12  |-  ( -.  x  =  y  -> 
( y  =  z  ->  A. x  y  =  z ) )

Detailed syntax breakdown of Axiom ax-12
StepHypRef Expression
1 vx . . . 4  set  x
2 vy . . . 4  set  y
31, 2weq 1626 . . 3  wff  x  =  y
43wn 3 . 2  wff  -.  x  =  y
5 vz . . . 4  set  z
62, 5weq 1626 . . 3  wff  y  =  z
76, 1wal 1529 . . 3  wff  A. x  y  =  z
86, 7wi 4 . 2  wff  ( y  =  z  ->  A. x  y  =  z )
94, 8wi 4 1  wff  ( -.  x  =  y  -> 
( y  =  z  ->  A. x  y  =  z ) )
Colors of variables: wff set class
This axiom is referenced by:  ax12v  1869
  Copyright terms: Public domain W3C validator