Full Idea
If the kinds there are depend not on the essences of the objects but on their observed distinguishing particulars, ...then for any kind that we think there is, it is possible that there are many underlying essences which are observably indistinguishable.
Gist of Idea
If kinds depend only on what can be observed, many underlying essences might produce the same kind
Source
Antony Eagle (Locke on Essences and Kinds [2005], IV)
A Reaction
Eagle is commenting on Locke's reliance on nominal essences. This seems to be the genuine problem with jadeite and nephrite (both taken to be 'jade'), or with 'fool's gold'. This isn't an objection to Locke; it just explains the role of science.