Full Idea
What ontological category a thing belongs to is not dependent on its inner nature, but dependent on what other things there are in the world, and this is a contingent matter.
Gist of Idea
A thing's ontological category depends on what else exists, so it is contingent
Source
Jan Westerhoff (Ontological Categories [2005], §89)
Book Reference
Westerhoff,Jan: 'Ontological Categories' [OUP 2005], p.218
A Reaction
This is aimed at those, like Wiggins, who claim that category is essential to a thing, and there is no possible world in which that things could belong to another category. Sounds good, till you try to come up with examples.