Full Idea
Set theory is at the very heart of mathematics; it may even be all there is to mathematics. The notion of set, however, seems quite contrary to the spirit of structuralism.
Gist of Idea
Sets seem basic to mathematics, but they don't suit structuralism
Source
James Robert Brown (Philosophy of Mathematics [1999], Ch. 4)
Book Reference
Brown,James Robert: 'Philosophy of Mathematics' [Routledge 2002], p.61
A Reaction
So much the worse for sets, I say. You can, for example, define ordinality in terms of sets, but that is no good if ordinality is basic to the nature of numbers, rather than a later addition.