Full Idea
We do not know that of any two different cardinal numbers one must be the greater.
Gist of Idea
We aren't sure if one cardinal number is always bigger than another
Source
Bertrand Russell (The Principles of Mathematics [1903], §300)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.323
A Reaction
This was 1903, and I don't know whether the situation has changed. I find this thought extremely mind-boggling, given that cardinals are supposed to answer the question 'how many?' Presumably they can't be identical either. See Burali-Forti.