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thomblake comments on The Featherless Biped - Less Wrong
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By Wittgenstein's time, there were already plenty of philosophers who thought definitions aren't quite captured by necessary and sufficient conditions.
And the recognition that the process that ordinary people went though had pretty much NOTHING in common with "necessary and sufficient conditions" was not made by philosophers.
Ordinary people struggle to decide whether dolphins are fish or penguins are birds. And they often get it wrong if they haven't been explicitly taught otherwise; even then, some still screw up their answers.
At what point do we say that the problem lies in the definition of a category? Since ordinary people have no especial use for the category "bird," it's unsurprising that they haven't nailed down characteristics that would allow such a use.
Categories that we need--that must reliably possess some characteristic(s) such that they are useful--tend to have strict necessary and sufficient conditions for inclusion. Categories that we use purely to simplify speech can get away with fuzzier definitions.
Is the dolphin really a fish? That depends: is that thing over there really a blegg?
The biological category of 'mammal' is quite well-defined, thank you.
And fuzzy definitions are fine until you're dealing with a case that lies in the penumbra, at which time it becomes a massive problem.
If a fuzzy definition becomes a massive problem, then that definition clearly wasn't in existence merely to simplify speech.
Regarding mammals, is there a use for the term that requires its inclusion of dolphins? Does the existence of sweat glands usefully separate mammals from other animals? After all, mammals in general share a variety of properties: most give live birth, most have hair, most are warm-blooded, etc.--but we admit to the category of mammals many animals that fail one or more of these criteria.
A well-defined but useless category (I am not arguing that "mammal" is such a category, as there may well be a biological use for it) may be pedagogically interesting but otherwise may merely confuse our understanding of thingspace.
".--but we admit to the category of mammals many animals that fail one or more of these criteria."
No, we don't. Dolphins have all of the required attributes to be considered mammals. If they didn't, we couldn't call them mammals any longer.
The definition of a mammal is simple: descent from the most recent common ancestor of all mammals. In practice, to avoid circularity, it is sufficient to take the MRCA of a few indisputable mammalian groups such as primates, rodents, carnivorans, ungulates, etc. to include all mammals.
This definition is useful because it turns out that there are many traits unique to mammals, and any given mammal will have almost all these traits. Many such traits are anatomical/biochemical/etc. (Many outwards traits like live birth or so-called "warm blood" aren't unique to mammals.)
However, even if this definition wasn't useful to us, the group Mammalia would still exist. It's a natural evolutionary group (clade) in phylogenetics, to which we merely give a name. (Edit: and cladistics is a natural way of classifying species (among other ways). By natural I mean a classification that tends to match common and unique traits of species in the same clade, and which is causally linked to the history of of the species and to predictions for their future, so that I would expect aliens to have a relatively high probability of using similar classifications.)
The precise clade referred to by the word Mammalia can change depending on context. It makes sense to ask whether borderline species like platypuses are mammals or a sister group of mammals. That's the fuzzy nature of any classification of real things. But the natural limits of the category "mammals" lie somewhere around the monotremes. A group which doesn't include dolphins is definitely not the group of all mammals.
But the MRCA of "indisputable" groups won't be an ancestor of basal groups like the monotremes or marsupials.
However, there's no dispute about including monotremes. The clade that excludes them is called the Theria. Likewise with the marsupials: the clade that excludes both them and the monotremes is the Eutheria. Every clade potentially has a name; Mammalia is just a particularly well known one.
Things get dicey if the evolutionary relationships are unclear, of course, or if some conventional group is recognised as not being a true clade.
You're right, of course. I was just pointing out that clades nest nicely. Whether you talk about Theria or Eutheria, the species included or excluded by the differences will be the most distantly related ones such as monotremes; but no clade anywhere similar in scope to Eutheria would be able to exclude dolphins. In that sense, it doesn't matter much which "indisputably" mammalian groups you take, their MRCA will be an ancestors of dolphins as well. For instance, the MRCA of humans and of cats is also an ancestor of dolphins.
carnivore- any animal that feeds on flesh; "Tyrannosaurus Rex was a large carnivore"
I suspect that "carnivore" there was meant to indicate "the order Carnivora".
Yes, I meant "Carnivorans", not "carnivores. Fixed and thanks :-)
Monotremes are mammals, yet do not share the property of giving live birth.
"Hair... may be greatly reduced in the Cetacea (i.e. dolphins), where it is found as a few scattered bristles about the lips or often present only in the young." W. J. Hamilton, American Mammals.
Heterocephalus glaber, the naked mole rat, is a mammal despite being poikilothermic.
As DanArmak notes, mammals are explicitly not the set of all species fulfilling a particular set of (external) criteria. They are defined by descent.
To the extent that this is useful, great. My point was merely that there is no external fact of the matter that requires drawing the boundaries where they currently are.
So when “ordinary people…get it wrong” by believing that dolphins are fish, it’s a little hard to blame them.
And many decidedly non-mammal animals do give live birth. Wikipedia lists "scorpions, some sharks, some snakes, [...] velvet worms [and] certain lizards".
What exactly do you mean when you say they believe that dolphins are fish? The question isn't about what to call a fish.
If they believe dolphins are like tuna in that they live entirely in the sea, have no hands and feet, swim with a thrashing tail, and eat other fish, then they are right.
And if they believe dolphins are like tuna in that they breath through gills and lay single-cell eggs into the water which grow into baby fish, then they are wrong. And I'd happily blame them for being wrong, because these are things that would be easy to get right with a little observation.
I'm suggesting that people see dolphins sharing the (easily-observable) characteristics they do with tuna and aren't especially interested in the respiratory or reproductive aspects--particularly because the category of "air-breathing fish" is wrong in some sense by construction.
They are also wrong on the biochemical differences, but insofar as fish are pretty much just "all the animals in the sea" to the general public, I'd say the scientific and the popular use of the word probably diverge.
Perhaps I'm being too much of an economist, but I figure that the actual need for most people to know most of these biological details is limited, and the fuzziness in the popular use of the word arises from a low-effort sweep of the observables.
For people who don't care in the least about dolphins (or fish), it makes sense. I shouldn't have assumed everyone shares a minimal interest in zoology.