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Definition df-cleq 2764
Description: Define the equality connective between classes. Definition 2.7 of [Quine] p. 18. Also Definition 4.5 of [TakeutiZaring] p. 13; Chapter 4 provides its justification and methods for eliminating it. Note that its elimination will not necessarily result in a single wff in the original language but possibly a "scheme" of wffs.

This is an example of a somewhat "risky" definition, meaning that it has a more complex than usual soundness justification (outside of Metamath), because it "overloads" or reuses the existing equality symbol rather than introducing a new symbol. This allows us to make statements that may not hold for the original symbol. For example, it permits us to deduce 𝑦 = 𝑧 ↔ ∀𝑥(𝑥𝑦𝑥𝑧), which is not a theorem of logic but rather presupposes the Axiom of Extensionality (see theorem axext4 2755). We therefore include this axiom as a hypothesis, so that the use of Extensionality is properly indicated.

See also comments under df-clab 2758, df-clel 2767, and abeq2 2881.

In the form of dfcleq 2765, this is called the "axiom of extensionality" by [Levy] p. 338, who treats the theory of classes as an extralogical extension to our logic and set theory axioms.

While the three class definitions df-clab 2758, df-cleq 2764, and df-clel 2767 are eliminable and conservative and thus meet the requirements for sound definitions, they are technically axioms in that they do not satisfy the requirements for the current definition checker. The proofs of conservativity require external justification that is beyond the scope of the definition checker.

For a general discussion of the theory of classes, see mmset.html#class. (Contributed by NM, 15-Sep-1993.)

Ref Expression
df-cleq.1 (∀𝑥(𝑥𝑦𝑥𝑧) → 𝑦 = 𝑧)
Ref Expression
df-cleq (𝐴 = 𝐵 ↔ ∀𝑥(𝑥𝐴𝑥𝐵))
Distinct variable groups:   𝑥,𝐴   𝑥,𝐵   𝑥,𝑦,𝑧
Allowed substitution hints:   𝐴(𝑦,𝑧)   𝐵(𝑦,𝑧)

Detailed syntax breakdown of Definition df-cleq
StepHypRef Expression
1 cA . . 3 class 𝐴
2 cB . . 3 class 𝐵
31, 2wceq 1631 . 2 wff 𝐴 = 𝐵
4 vx . . . . . 6 setvar 𝑥
54cv 1630 . . . . 5 class 𝑥
65, 1wcel 2145 . . . 4 wff 𝑥𝐴
75, 2wcel 2145 . . . 4 wff 𝑥𝐵
86, 7wb 196 . . 3 wff (𝑥𝐴𝑥𝐵)
98, 4wal 1629 . 2 wff 𝑥(𝑥𝐴𝑥𝐵)
103, 9wb 196 1 wff (𝐴 = 𝐵 ↔ ∀𝑥(𝑥𝐴𝑥𝐵))
Colors of variables: wff setvar class
This definition is referenced by:  dfcleq  2765  bj-ax9  33218  bj-ax9-2  33219
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