Full Idea
Mathematics is today thought of as the study of abstract structure, not the study of quantity. That point of view arose directly out of the development of the set-theoretic notion of abstract structure.
Gist of Idea
Mathematics is nowadays (thanks to set theory) regarded as the study of structure, not of quantity
Source
Shaughan Lavine (Understanding the Infinite [1994], III.2)
Book Reference
Lavine,Shaughan: 'Understanding the Infinite' [Harvard 1994], p.47
A Reaction
It sounds as if Structuralism, which is a controversial view in philosophy, is a fait accompli among mathematicians.