Full Idea
In the Kripke-Putnam view, it is very difficult for anyone except nuclear physicists to pick out natural kinds, since everything else is made out of compounds of different isotopes.
Clarification
The Kripke-Putnam view depends on microstructure, not properties
Gist of Idea
In the Kripke-Putnam view only nuclear physicists can know natural kinds
Source
Alexander Bird (Philosophy of Science [1998], Ch.3)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.107
A Reaction
The concept of a rigid 'natural kind' does not have to be sacred. Tin might be considered a natural kind, despite having 21 isotopes. What matters is protons, not the neutrons.