“Natural Kinds” Philosophy 63 (1988) pp 29 – 42
The problem of Natural Kinds is that there is no agreement anywhere about a doctrine for how to determine natural kinds. Certain paradigm examples have agreement: the kinds oak, stickleback and gold are natural kinds, and the kinds table, nation, and banknote are not. Wilkinson identifies two conditions which must be fulfilled to categorize a natural kind:
1) the notion of a natural kind must be tied to that of real essence.
2) 2) members of natural kinds, and the corresponding real essences, lend themselves to scientific investigation.
Natural kind predicates are also inductively predictable. It is easy to predict the behavior of the lump of stuff in front of me if I am able to determine it is oak. Alternatively, it is impossible to inductively predict the behavior of a nation. Also, any single object/entity might belong to one natural kind and numerous non-natural kinds. This does not effect the concept of inductive predictability as you are making your inductive predictions based on membership to a natural kind.
Distinguishing Between Natural Kinds and Non-Natural Kinds
A natural kind relates directly to the science that studies the real essence. For instance, ‘clouds’ do not have real essence, thereby a meteorologist does not study a real essence as such. Gases and water vapor do have real essence and so the chemist studies more precisely the natural kind. Likewise, a geographer or geologist might study a cliff, but the cliff does not have real essence. Instead it is the rock or granite, the realm of the physicist/chemist. See Locke on real and nominal essences.