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Full Idea
There cannot be any borderline cases between the real essences of different natural kinds because, if there were, the distinctions between the kinds would be superficial, like the blue/green distinction.
Gist of Idea
If there are borderline cases between natural kinds, that makes them superficial
Source
Brian Ellis (Scientific Essentialism [2001], 1.05)
Book Ref
Ellis,Brian: 'Scientific Essentialism' [CUP 2007], p.31
A Reaction
His particular target here is biological natural kinds, in which he doesn't believe, because they blur across time, in the evolutionary process. Personally I am inclined to relax the notion of a natural kind, otherwise they are too basic to explain.
Related Ideas
Idea 16750 If there are just arrangements of corpuscles, where are the boundaries between substances? [Pasnau]
Idea 17410 Moseley showed the elements progress in units, and thereby clearly identified the gaps [Scerri]
| 10398 | Natural kinds are not special; they are just well-defined resemblance collections [Abelard, by King,P] |
| 13575 | If there are borderline cases between natural kinds, that makes them superficial [Ellis] |
| 16954 | Generalised talk of 'natural kinds' is unfortunate, as they vary too much [Dummett] |
| 6765 | Nominal essence may well be neither necessary nor sufficient for a natural kind [Kripke, by Bird] |
| 15299 | Species do not have enough constancy to be natural kinds [Harré/Madden] |
| 17375 | Natural kinds are decided entirely by the intentions of our classification [Dupré] |
| 17380 | Wales may count as fish [Dupré] |
| 17382 | Cooks, unlike scientists, distinguish garlic from onions [Dupré] |
| 17379 | Borders between species are much less clear in vegetables than among animals [Dupré] |
| 17384 | Even atoms of an element differ, in the energy levels of their electrons [Dupré] |
| 17387 | Ecologists favour classifying by niche, even though that can clash with genealogy [Dupré] |
| 10351 | Natural kinds are social institutions [Kusch] |