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Classes, Categories and Words
First, let us agree that a class is the same as a category. Both words are in common use, but they both mean a group of items bound together by having certain features in common. For example, sheets are things whose width is many times larger than their thickness, and that category includes sheets of paper, sheets on a bed and sheets of water. The items in the class may be very different in many ways, but as long as they possess the characteristics or properties that establish the class, they belong in it.
There is a famous difficulty about, for example, a dog. If you think a dog has four legs, a tail and a good nose for sniffing, you are in trouble when you meet a dog with three legs, or no tail, or no sense of smell, though still clearly a dog. Rather than list the characters an animal must have to be a dog, an alternative is to conceive a prototype --- a kind of ideal dog --- and to say that any item that resembles the Dog Prototype more than it resembles any other prototypes you have in mind is to be called a dog. The literature on how to categorize, logical ways of categorizing, using fuzzy logic etc. is vast; "there is no topic, however complicated, that when approached from a suitable point of view, can't be made even more complicated." But for our purposes, whether we assign an item to a class by its characteristics or by resemblance to a prototype doesn't matter. What matters is that language is based on classes that are based on matching, and that matching is a pleasure. I have proposed on other pages that pleasure-from-matching is a widespread effect, and if you think for a moment how much of one's mental life involves words, you see just a part of what I mean.
(Is that true, that language is based on classes? I will give just one example, the slightly abstract word "above". My nose is above my mouth, my knee is above my foot, and the distant clouds are above the distant hills. The three relations mentioned are all examples of "above," and "above" signifies the particular relation that is common to all three examples. This is another topic that can be made complicated, but my example covers a large proportion of the words we use.)
Now I can bring in a favorite quotation from more than 100 years ago, from William James in 1892: "... (non-human animals) never extract characters for the mere fun of the thing, as men do." By "extracting characters", he means isolating just those features that make a sheet a sheet, or whatever --- and he recognizes that there is pleasure in doing the job. Regardless of material benefit, a person will form categories just for fun; a person will form categories of more and more abstract kinds, until we have words for honesty and fondness and indissolubility. The ancient Greeks thought carefully about categories, but the pleasure attached to forming them has been much less noticed, even after William James remarked on it.