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scotherns comments on The Featherless Biped - Less Wrong
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To go on a bit of a tangent...
Upon seeing this, I decided to try to come up with my best guess as to what the necessary features of a bird are. So I came up with the birdiest non-bird I could think of, thinking qualitatively: why is a bat not a bird? I realized that bats don't lay eggs, but platypuses, a mammal, do; since I couldn't think of a mammal that both lays eggs and flies, but I could think of a mammal that does each, I postulated a hypothetical mammal that does lay eggs and fly, and compared it to a hypothetical bird with fine, hair-like feathers, and pondered what would distinguish these; out of somewhere, the idea came to me that birds' lungs are different somehow. So that was one of my distinguishing features: birds have a certain type of lung.
I also considered the birdiest non-birds I could think of, thinking taxonomically; these were the reptiles. It was easier to come up with a distinguishing feature here: birds are warm-blooded.
So, my thought left me with this best definition: "A bird is a warm-blooded organism with a certain type of lung." Looking up on Wikipedia what type of lung birds have, I was able to refine this into this: "A bird is a warm-blooded organism with circulatory lungs." How close did I come?
So if I removed the lungs of chicken, you would no longer consider it a bird? Or if I surgically modified some other creature (e.g. a pig) to have circulatory lungs, you would consider this to be a bird?
This kind of argument is why it is pretty difficult to come up with a comprehensive set of features for a broad category like 'bird'. Often the best you can do is produce a set of examples demonstrating the category. Humans are pretty good at such pattern recognition from a set of data.
Like a lot of things, it is hard to define, but you know it when you see it :-)
Different people's concepts of "bird" agree on most real-world examples, but I see no reason why they should agree on all conceivable hypothetical examples, so the task of "defining" a word is futile.
Warrigal gave a good recognition algorithm: it inspects a small subset of properties and gives an answer that accords with our judgment in most real-world cases. That's about as far as one can or should go when "defining" something outside of mathematics.
Even though no bird, in the history of the world, has ever been recognised using it ?
When someone proposes a new algorithm, "this algorithm has never been used" doesn't sound to me like a valid critique. More substantively, Cuvier proposed similar outlandish-sounding algorithms tuned to recognizing animals by teeth and bone fragments, which have enjoyed widespread use ever since.
A small anecdote: one of Cuvier's students once dressed in a devil's costume and entered his room at night to scare him. Cuvier opened his eyes, said "Horns? Hooves? You can't eat me, you're a herbivore" and went back to sleep.
Heh.
Plenty of herbivores can still do serious damage when they're annoyed, though. ;)
Well, if you remove the lungs of a chicken, its species is still a species whose members typically have circulatory lungs. What I was wondering is whether there are any species of bird that are not warm-blooded with circulatory lungs, or if there are any species of anything else that are.