Full Idea
While a shape or a direction is necessarily of something, games, musical compositions or dance routines are not of anything at all.
Gist of Idea
Shapes and directions are of something, but games and musical compositions are not
Source
Bob Hale (Abstract Objects [1987], Ch.3.II)
Book Reference
Hale,Bob: 'Abstract Objects' [Blackwell 1987], p.55
A Reaction
This seems important, because Frege's abstraction principle works nicely for abstractions 'of' some objects, but is not so clear for abstracta that are sui generis.