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Axiom ax-ext 1877
Description: Axiom of Extensionality. An axiom of New Foundations set theory. It states that two sets are identical if they contain the same elements. Axiom Ext of [BellMachover] p. 461.

Set theory can also be formulated with a single primitive predicate on top of traditional predicate calculus without equality. In that case the Axiom of Extensionality becomes , and equality is defined as . All of the usual axioms of equality then become theorems of set theory. See, for example, Axiom 1 of [TakeutiZaring] p. 8.

To use the above "equality-free" version of Extensionality with Metamath's logical axioms, we would rewrite ax-8 1402 through ax-16 1606 with equality expanded according to the above definition. Some of those axioms could be proved from set theory and would be redundant. Not all of them are redundant, since our axioms of predicate calculus make essential use of equality for the proper substitution that is a primitive notion in traditional predicate calculus. A study of such an axiomatization would be an interesting project for someone exploring the foundations of logic.

General remarks: Our set theory axioms are presented using defined connectives (, , etc.) for convenience. However, it is implicitly understood that the actual axioms use only the primitive connectives , , , , and . It is straightforward to establish the equivalence between the actual axioms and the ones we display, and we will not do so.

It is important to understand that strictly speaking, all of our set theory axioms are really schemes that represent an infinite number of actual axioms. This is inherent in the design of Metamath ("metavariable math"), which manipulates only metavariables. For example, the metavariable in ax-ext 1877 can represent any actual variable v1, v2, v3,... . Distinct variable restrictions ($d) prevent us from substituting say v1 for both and . This is in contrast to typical textbook presentations that present actual axioms. In practice, though, the theorems and proofs are essentially the same. The $d restrictions make each of the the infinite axioms generated by the ax-ext 1877 scheme exactly logically equivalent to each other and in particular to the actual axiom of the textbook version.

Ref Expression
Distinct variable group:   ,,

Detailed syntax breakdown of Axiom ax-ext
StepHypRef Expression
1 vz . . . . 5
2 vx . . . . 5
31, 2wel 1401 . . . 4
4 vy . . . . 5
51, 4wel 1401 . . . 4
63, 5wb 173 . . 3
76, 1wal 1322 . 2
82, 4weq 1399 . 2
97, 8wi 4 1
Colors of variables: wff set class
This axiom is referenced by:  axext2  1878  axext3  1879  bm1.1  1881  dfcleq  1889
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