I’ve known for a long time that humans are fish. Most people know the slightly less specific claim “There is No Such Thing as a Fish” (which has a wonderful podcast named after it.) The idea coming from cladistic taxonomy where the classification of species was redone with a focus on ancestry. (As opposed to Linnaean Taxonomy which was based on shared physical characteristics.)
The idea that you might be a fish strikes you as weird for several reasons. The most important of which is that we tend to learn basically/generally/kinda what words mean without ever really checking to figure out exactly what they mean. The really interesting thing is that most of us are able to go through our lives without ever even realizing that we don’t really know the meaning of tons of what comes out of our mouths. But you often stumble across it in weird smart-alecky facts like a tomato is a fruit, not a vegetable.
As a result the words we use in everyday conversation don’t define rigid boundaries, but kind of vague fuzzy areas. So there is a firm core of what it means to be a vegetable, like a carrot or a celery stalk. But a clove of garlic doesn’t feel so vegetably. And a mushroom is actually from an entirely separate kingdom. Yet for the purposes of cooking dinner, they group rather nicely in the same section of the grocery store.
There are two ways to define words: descriptive and prescriptive. For the most part we are descriptive language users. A word means the thing that most people are talking about when they use that word most of the time. And as a result words change their meaning over time and if they change slowly enough we don’t notice and don’t care.
But for those that like to think that words ought to have rigid definitions that should be well defined and shouldn’t change you run into problems. Most words have fuzzy definitions because the world is fuzzy definitions. There isn’t a hard line between vegetable and non vegetable built into the fabric of the universe. But if you place rigid definitions in place, then some words wind up in categories you didn’t plan. Like the idea that tomatoes and mushrooms aren’t vegetables, or peanuts aren’t nuts.
The other thing we don’t really like is nesting structures or overlapping Venn Diagrams. In grade 10 studying analytic geometry, we had a long discussion about whether or not a square was a rectangle. A square has all the properties of a rectangle plus one extra one: the length and width are equal. But because we learn our shapes off a list (circle, square, triangle, rectangle) we really see squares as a separate thing rather than a subset of the class of shapes called rectangles. Like a kid who grows up and leaves home, squares take on enough properties that they cleave off their parent class and stop being part of it. But classification mostly relies on taking groups and cutting them into subsets, so the idea that you can be part of a subset, add extra properties and stop being part of that subset makes classification a nightmare.
This gets really upsetting for us as we think of ourselves in a certain way, they try to hash out a rigid definition and realize we aren’t the things we thought we were.