Full Idea
The 'magical' conception of structural universals says 'simple' must be distinguished from 'atomic'. A structural universal is never simple; it involves other, simpler, universals, but it is mereologically atomic. The other universals are not its parts.
Gist of Idea
The 'magical' view of structural universals says they are atoms, even though they have parts
Source
David Lewis (Against Structural Universals [1986], 'The magical')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.100
A Reaction
Hence the 'magic' is for it to be an indissoluble unity, while acknowledging that it has parts. Personally I don't see much problem with this view, since universals already perform the magical feat of being 'instantiated', whatever that means.
Related Idea
Idea 15449 If 'methane' is an atomic structural universal, it has nothing to connect it to its carbon universals [Lewis]