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Full Idea
It is not at present known whether, of two different infinite numbers, one must be greater and the other less.
Gist of Idea
We do not currently know whether, of two infinite numbers, one must be greater than the other
Source
Bertrand Russell (The Principles of Mathematics [1903], §118)
Book Ref
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.122
A Reaction
This must refer to cardinal numbers, as ordinal numbers have an order. The point is that the proper subset is equal to the set (according to Dedekind).