Full Idea
You can think of an ordinal number as being defined by the set that comes before it, so, in the non-negative integers, ordinal 5 is defined as {0, 1, 2, 3, 4}.
Gist of Idea
An ordinal number is defined by the set that comes before it
Source
Brian Clegg (Infinity: Quest to Think the Unthinkable [2003], Ch.13)
Book Reference
Clegg,Brian: 'Infinity' [Robinson 2003], p.169