Full Idea
The 'Van Inwagen Problem' for structuralism is of explaining how a mathematical relation (such as set membership, or the ratios of an ellipse) can fit into one of the three scholastics types of relations: are they internal, external, or intrinsic?
Gist of Idea
How can mathematical relations be either internal, or external, or intrinsic?
Source
John P. Burgess (Review of Chihara 'Struct. Accnt of Maths' [2005], §5)
A Reaction
The difficulty is that mathematical objects seem to need intrinsic properties to get any of these three versions off the ground (which was Russell's complaint against structures).