Full Idea
The question of what a pre-scientific term extends over is extremely difficult for a Putnam-style externalist to answer. …There seems no good reason to assume that they extend over natural kinds ('whale', 'cat', 'water').
Gist of Idea
Externalists can't assume old words refer to modern natural kinds
Source
Gabriel M.A. Segal (A Slim Book about Narrow Content [2000], 5.1)
Book Reference
Segal,Gabriel M.A.: 'A Slim Book about Narrow Content' [MIT 2000], p.131
A Reaction
The assumption seems to be that they used to extend over descriptions, and now they extend over essences, or expert references. This can't be right. They have never changed, but now contain fewer errors.