Full Idea
Structuralists give a historical account of why the 'same' number occupies different structures. Numbers are equivalent rather than identical. 8 is the immediate predecessor of 9 in the whole numbers, but in the rationals 9 has no predecessor.
Clarification
For 'rational' numbers see Idea 8668
Gist of Idea
In structuralism the number 8 is not quite the same in different structures, only equivalent
Source
Michèle Friend (Introducing the Philosophy of Mathematics [2007], 4.4)
Book Reference
Friend,Michèle: 'Introducing the Philosophy of Mathematics' [Acumen 2007], p.93
A Reaction
I don't become a different person if I move from a detached house to a terraced house. This suggests that 8 can't be entirely defined by its relations, and yet it is hard to see what its intrinsic nature could be, apart from the units which compose it.