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Fingers and thumbs

Logicians, mathematicians, linguists, and other overly smart people often have problems with something your average hick can easily express and comprehend: the subset of a set that excludes another subset.

That’s a mouthful but here are two super clear examples:

Someone might say “humans and animals” but we overly smart people will fall over ourselves saying “humans are animals!” at risk of missing the original point the speaker was trying to make.

“Fingers and thumbs” is another example.

Linguist Ruth Kempson did some work sorting this out in the seventies, and the phenomena has had a couple of different names throughout the years (like the confusing term “autohyponym” from 1984). The one I like is “vertical polysemy”.

That’s great because knowing that sparrows, giraffes, bunnies, and baseball players technically are all subsets of fish would make it kind of hard to talk about normal fish. That’s right. A dolphin (or a human for that matter) is more closely related to a guppy than a shark is, since dolphins, humans, and guppies all are Osteichthyes probably better known as “bony fish” while sharks are Chondrichthyes. But there is also a word “fish” (spelled and pronounced exactly the same, thanks to vertical polysemy rather than homonymy) that just means normal fish and by that definition sharks and guppies are fish and giraffes aren’t. Or I’d like to see your aquarium.

Conclusion: just remember that words mean different things in different contexts and you’ll be alright. Watch out for equivocation fallacies, both for falling for them when others use ‘em (perhaps inadvertently) and for using them yourself.