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 there are border cases like Ceres, but I think there is a natural category (whatever that means!) that includes the rocky planets, gas giants and Kuiper Belt objects that does not include (most) asteroids and comets.
On the fish example, I claim that the definition of “fish” that includes the modern definition of fish union the cetaceans is a perfectly valid natural category, and that this is therefore an intensional definition. “Fish” are all things that live in the water, have finlike or flipperlike appendages and are vaguely hydrodynamic. The fact that such things do not all share a comment descent* is immaterial to the fact that they look the same and act the same at first glance. As human knowledge has increased, we have made a distinction between fish and things that look like fish but aren’t, but we reasonably could have kept the original definition of fish and called the scientific concept something else, say “piscoids”.
*well, actually they do, but you know approximately what I mean.
Nitpick: if in your definition of fish, you mean that they need to both have fins or flippers and be (at least) vaguely hydrodynamic, I don’t think seahorses and puffer fish qualify.
Yes, but neither fish nor (fish union cetaceans) is monphylatic. The decent tree rooted at the last common ancestor of fish also contains tetrapods and decent tree rooted at the last common ancestor of tetrapods contains the cetaceans.
I am not any sort of biologist, so I am unclear on the terminological technicalties, which is why I handwaved this in my post above.
I’m inclined to agree. Having a name for ‘things that naturally swim around in the water, etc’ is perfectly reasonable and practical. It is in no way a nitwit game.
“[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?:
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 there are border cases like Ceres, but I think there is a natural category (whatever that means!) that includes the rocky planets, gas giants and Kuiper Belt objects that does not include (most) asteroids and comets.
On the fish example, I claim that the definition of “fish” that includes the modern definition of fish union the cetaceans is a perfectly valid natural category, and that this is therefore an intensional definition. “Fish” are all things that live in the water, have finlike or flipperlike appendages and are vaguely hydrodynamic. The fact that such things do not all share a comment descent* is immaterial to the fact that they look the same and act the same at first glance. As human knowledge has increased, we have made a distinction between fish and things that look like fish but aren’t, but we reasonably could have kept the original definition of fish and called the scientific concept something else, say “piscoids”.
*well, actually they do, but you know approximately what I mean.
Nitpick: if in your definition of fish, you mean that they need to both have fins or flippers and be (at least) vaguely hydrodynamic, I don’t think seahorses and puffer fish qualify.
The usual term is “monophyletic”.
Yes, but neither fish nor (fish union cetaceans) is monphylatic. The decent tree rooted at the last common ancestor of fish also contains tetrapods and decent tree rooted at the last common ancestor of tetrapods contains the cetaceans.
I am not any sort of biologist, so I am unclear on the terminological technicalties, which is why I handwaved this in my post above.
Fish are a paraphyletic group.
I’m inclined to agree. Having a name for ‘things that naturally swim around in the water, etc’ is perfectly reasonable and practical. It is in no way a nitwit game.