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It is not the case that In standard ZFC set theory, the class of all ordinals (On) is a proper class, not a set, so it admits no order type as an ordinal.
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Reasons For
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Reason for
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1.
One can coherently define an 'absolute order type' of On in extended frameworks (like Morse-Kelley set theory) without contradiction.
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2.
The restriction to sets for order types is a convention of ZFC, not a metaphysical fact about ordering or well-foundedness.
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3.
Proper classes possess well-ordering and transitivity; denying them an order type seems arbitrary rather than principled.
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Reasons Against
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Reason against
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1.
Cantor's Burali-Forti paradox shows that assuming On is a set leads to contradiction, so ZFC correctly excludes it.
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2.
Order types are defined as isomorphism classes of well-ordered sets; only sets can be members of such classes in ZFC.
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3.
Proper classes lack cardinality and cannot be ranked by ordinals without collapsing the cumulative hierarchy.
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