"[E]ntirely a definitional" question does not mean "arbitrary and trivial" - some definitions are just wrong. EY mentions the classic example in Where to Draw the Boundary?:
Once upon a time it was thought that the word "fish" included dolphins. Now you could play the oh-so-clever arguer, and say, "The list: {Salmon, guppies, sharks, dolphins, trout} is just a list - you can't say that a list is wrong. I can prove in set theory that this list exists. So my definition of fish, which is simply this extensional list, cannot possibly be 'wrong' as you claim."
Or you could stop playing nitwit games and admit that dolphins don't belong on the fish list.
Honestly, it would make the most sense to draw four lists, like the Hayden Planetarium did, with rocky planets, asteroids, gas giants, and Kuiper Belt objects each in their own category, but it is obviously wrong to include everything from Box 1 and Box 3 and one thing from Box 4. The only reason it was done is because they didn't know better and didn't want to change until they had to.
You (well, EY) make a good point, but I think neither the Pluto remark nor the fish one is actually an example of this.
In the case of Pluto, the transNeptunians and the other planets seem to belong in a category that the asteroids don't. They're big and round! Moreover, they presumably underwent a formation process that the asteroid belt failed too complete in the same way (or whatever the current theory of formation of the asteroid belt is; I think that it involves failure to form a "planet" due to tidal forces from Jupiter?). Of course th...
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