Natural kinds with an example.

[ParaclitosLogos]

We have hypothesis H1: All emeralds are green
We have hypothesis H2: All emeralds are grue

Where Grue:def.  green up until 1 jan 1971, and, blue afterwards.
Where Bleen:def. blue up until 1 jan 1971, and green afterwards
And then, we have that Green is grue up until 1 jan 1971, and, bleen afterwards
All observations up to 1 jan 1971 support both H1 and H2.

What makes the difference?
It is argued that green is the more “entrenched” term.

IOW, (“entrenched” is) that the history of its sociological linguistic use, tell us that it (and any other co-extensional term) has been used plenty in successful inductive inferences, and thus, it is a good candidate to a natural kind (not a socio-linguistic construction).

Natural kinds are genuinely explanatory.
1.
     a. Non-normative: Rover has 4 legs because Rover is a dog. (E.g., biology and physics.)
     b. Normative: Rover should have 4 legs because Rover is a dog. (E.g., biology but not physics.)

2. Natural kinds enable inductive inference.
     - Rover and Fido are dogs. Rover has 4 legs. That’s evidence that  Fido does, too.

3. Natural kinds capture genuine similarity... not constituted by how we contingently think about the items under consideration.