Full Idea
A general theory of causation in an area is possible only if the kinds of entities under investigation can reasonably be assumed to belong to natural kinds.
Gist of Idea
A general theory of causation is only possible in an area if natural kinds are involved
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.7)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.161
A Reaction
Human beings will be a problem, and also different levels of natural kinds (e.g. a chemical and an organism). 'Natural kind' is a very loose concept. He is referring to scientific, rather than philosophical, theories, I presume.