Full Idea
Ontological categories should not be confused with natural kinds: for natural kinds can only be differentiated in a principled way relative to an accepted framework of ontological categories.
Gist of Idea
Ontological categories are not natural kinds: the latter can only be distinguished using the former
Source
E.J. Lowe (The Possibility of Metaphysics [1998], 8.2)
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.179
A Reaction
I presume that the natural kinds are likely to be contingent facts about the actual world (though they may entail necessary laws), whereas I like to think, unfashionably, that categories aim at deconstructing the mind of God (roughly).