What we decide is the Basic Level𓇯 of categorization depends on what distinctions it's typically important for us to attend to. "Importance" is not so much due to how the world *is* as how we habitually interact with it.
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A handle isn’t just a long, thin thing attached to another part or to some whole: a handle has to be grippable. **By us**. The difference between a table and a chair isn’t just that a table has no back; it’s that the flat part of a chair is the right size **for us** to sit on, and the seat is set at the right height **for us** to flop down into.
**Learning basic categories depends on interaction.** Consider that the part-and-whole relationships within a cat aren't wildly different from those within a dog: four legs, head on a flexible neck, similar shaped heads, tail, and so on. Why aren’t they lumped together as the basic category of “pet”?
The answer is simple, in one sense: parents don’t teach children about pets as a category early on: they specifically point out and name “doggie” and “kitty.” But what kind of reason is that? Parents don’t use “pet” because *their* parents didn’t use “pet”? *Why* didn’t any of those parents use “pet”? And what helps the child distinguish cats from dogs?
So consider a small child’s prototypical interaction with a cat. It very likely involves a nearby guardian luring the solitary cat closer, then telling the child “now, be very gentle petting her.” Cats don’t like roughhousing.
In contrast, a child on my street will almost never see an unaccompanied dog. City ordinance says dogs need to be leashed. So that’s one difference from cats: dogs come attached to people. And the words the child's guardian speaks are different: they’re addressed to the dog’s servant, and probably sound like “Is it OK…”, to which the servant will say something like “Oh sure, she’s very good with kids.” Such a dog will also likely be very much bouncier than a cat and interact completely differently.
This understanding of how the two animals are encountered is part of their basic categories because it’s important to decide quickly how to interact with that four-legged, two-eared, thing-with-a-face over there. The category “Pet” wouldn’t be helpful. It’s such expectations that drive the closer examination of parts and part-whole relationships that leads to cats being so obviously different from dogs as a matter of its Basic Level Gestalt𓇯.
That is, in the terminology I prefer, it’s not the visible structure of the animal that’s important, with associations to behavior tacked on. **The *how* of interacting comes first, and parts are learned in support of that.**