Suppose that somewhere in the galaxy there is a planet where there are large six-legged animals with an inner supportive structure, that evolved completely independently of any forms of life on earth and whose genetic structure is not based on DNA but another molecule. What I said seems perfectly possible. But it is impossible if animals are simply the members of the kingdom Animalia, since the six-legged animals on that planet are neither DNA-based nor genetically connected to the animalia on earth.
On the other hand, the supposition that somewhere (maybe in another universe) there is water that does not have H2O in it is an impossible one. So is the supposition that there are horses without DNA.
So the kind animal is disanalogous to the kinds water and horse. The kind water is properly identified with a chemical kind, H2O, and the kind horse is properly identified with a biological species, Equus ferus. But the kind animal does not seem to be properly identified with any biological kind.
One can have DNA-based animals and non-DNA-based animals. If the Venus fly-trap evolved the ability to move from place to place following its prey, it would be an animal, but still a member of Plantae. Animals are characterized largely functionally, albeit not purely functionally, but also in reference to the function of their embodiment—there cannot be any animals that are unembodied.
Is animal a genuine natural kind? Or is it a non-natural kind, constructed in the light of our species’ subjective interests? I don’t know. I take seriously, though, the possibility that there is an "Aristotelian" philosophical categorization that goes across biological categories.
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We already have an example of something that classifies differently under Aristotle's system than under ours: coral, which would have been a plant to Aristotle's system and is an animal (with a symbiotic microscopic component) under the modern one.
But just as a synthetic material fur coat is still a fur coat, there seems to be no problem with using "animal" as a familiar term to describe the aliens even if biologically they would not share a kingdom in classification. It's only if we drew inappropriate conclusions from the "animal" term being used that things would be expected to go sideways.
Alex,
It seems to me that the word "animal" is used to mean different things - and with different referents - in different contexts. In a documentary, the referent is (probably) the members of the kingdom Animalia.
Regarding "horse" and other animals, it seems to me that the referent is also picked on the basis of both our psychology and the stuff we encounter and do not have sharp boundaries in general.
For example, let's say that things with the same DNA and looks as horses evolves on another planet, say Earth-2 (one just needs the universe to be sufficiently large, not necessarily infinite), alongside things like humans. They also use the word "horse". Are those horses? If not, then the kind "horse" seems to be a "location-kind" so to speak: the stuff they're made of is not relevant, neither the behavior, etc., but rather, on what planet they (or their ancestors) are.
On the other hand, let's say those are horses too. But on another planet, Earth 3, their DNA is just very slightly different on average. Would they still be horses? It seems they would. Not all horses have the same DNA on Earth, either. But slight changes would not make them non-horses. In fact, even a single mutation here on Earth would not result in a non-horse. On Earth 4, they are slightly more different, and so on. Eventually, there is n, such that the things called "horse" on Earth n are not horses. They're n-horses. But it seems to me that:
1. There may not be a fact of the matter as to which the minimum such n is; the word "horse" is simply not precise enough to be properly used in that context.
2. It may well be that for some 1<j<n, j-horses on Earth j are both horses and n-horses.
Are they all natural kinds that overlap?
With living things, a natural kind would also generally be a common clade or kindred (ie the individuals would be related to one another in some way by ancestral descent). So, the alien horse is not going to be the same natural kind as the Earth horse.
William,
But if so (I'm not sure what Alex thinks about that), that seems to also depend on the psychology of the agent classifying them. Some non-human aliens might have a word "alien-horse" and a word "alien-natural kind" such that things on different planets can be alien-horses, and be part of an alien-natural kind that does not require common ancestry.
Have you read David Oderberg's Real Essentialism? Chapter 9 argues for an Aristotelianism about biological kinds based on morphology rather than cladistics, and he discusses an example similar to your alien animals (among others).
Thought experiments involving the ad hoc resemblance of hypothetical aliens also seem quite dependent on our psychology.
William,
In a sense, yes, but I'm not sure what you're trying to get at.
My point is that natural kinds, at least according to the views I'm familiar with, are supposed to be at least independent of the psychology of the agent making the classification; somehow, they're supposed to "carve nature at its joints" so to speak. Ancestry-based kinds do not seem to meet the criterion. This isn't to say that something meets said criterion; I take no stance on that, but at least, it seems that sort of kinds do not meet it.
A six-legged horse would definately increase my shoing costs. May lower lameness issues though. Instead of the weight being supported by four legs, the weight is supported on six. Riding at a jog or a lope though would be a whole new ball game.
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