Politesse said:
It would be "reasonable", but by no means an actual scientific definition of a duck. If you're going to cling to nature for support, you should be bound by what nature can actually establish. Don't insist that science is on your side, then retreat to "reason" when science turns out not to be saying what you want it to.
First, it would be reasonable because in that context, the claim is not a universal one, but a true and warranted claim. Of course, it's not the definition of a duck - scientific or otherwise. Rather, it's information about ducks, which is not the same as giving a definition of the word.
Second, I do not "retreat" to "reason", because I never made a claim that science is on my side - just facts - and because that would not be a retreat.
Politesse said:
Sure, horses have at least two sexes, and a whole bunch of intermediary cases, like just about every sexually reproducing vertebrate. "Normal vs Abnormal" is a value judgement arbitrarily applied, and is irrelevant in any case to whether dyadic perceptions of sex are accurate. Calling something abnormal does not cause it to suddenly cease to exist.
Actually, "normal vs. abnormal" is an assessment properly made, not arbitrarily applied. Surely, 4-legged ducks are abnormal. And the same goes to the horse in question. Obviously, causing something 'abnormal' does not cause it to suddenly cease to exist. But calling an assessment that something is abnormal a "value judgement arbitrarily applied" does not cause it to cease to be true, either. The same goes for saying that something malfunctioned, that there was something wrong with the development, etc.
Politesse said:
I challenge in any case whether a genetic abnormality like an extra leg is in the same class of things as a property of sexual diversity that seems to exist within the karyotypic diversity of most animals, but even if it is, then in both cases a scientist would be an idiot to say that "All ducks have either two or one legs" given that there is evidence for exceptions to that would-be rule.
I'd say that a person attacking the view that holds that "All ducks have either two or one legs" as the position of all of those who claim that ducks are two-legged animals would be misrepresenting the position of nearly all if not all of the people who say that ducks are two-legged animals.
As to your challenge, here you talk about a "genetic abnormality", so you seem to agree it's abnormal. But why do you think it's genetic? Maybe it is, or maybe some other kind of developmental anomaly (e.g., absorbed twin). I don't see any good evidence either way in this case. But I haven't looked into the details, so maybe there is good evidence that it's genetic. Why would its being genetic be important?
That aside, given that sex is found in most animals and it's a very complex thing, it's unsurprising that developmental abnormalities involving sex are also found in many or even most animals. But why would it not be in the same category? Is it because of the numbers of abnormal cases?
Politesse said:
And if ducks had third legs at birth at the same rate of 1 in 2000-4000 or so that typify intersex individuals, an ornithologist would certainly prefer to say that "most ducks are bipedal" then mention the very common exception case.
That looks very uncommon to me. How do you measure "very common"?
Humans have color vision, even if 1 in 30000 are born with achromatopsia (
https://en.wikipedia.org/wiki/Achromatopsia ). Sure, that's about 10 times less. But then again, if it's about numbers why is the specific 1 in 2000-4000 particularly important, and not, say, 300 or 30000?
On the other hand, color blindness among humans is far more common than intersexuality. In fact, either protanopia or deuteranopia is, on its own, far more common than intersexuality. Incidentally, in both cases, it's far more common among humans because of how common it is among human males, and males make up roughly 50% of the human population. It is rare in females, who roughly make up another 50%, but 1 half of the rate among males is still over ten times as high as 1 in 2000 (for either protanopia or deuteranopia alone).
Regarding the ornithologist, it would depend on what the ornithologist is trying to get at. I'm inclined to think he would prefer to say that ducks are bipedal in most or nearly all contexts, and that he would say that 4-legged ducks are abnormal cases if asked - unless perhaps some ideology (some variant of leftism, probably) has taken hold of the social circle of ornithologists, which may well be the case going by pronouncement by scientific bodies, many Western scientists, etc., on a number of issues (e.g., sex, race, etc.).