Yesterday, we saw that in Japan, blood types have taken the place of astrology—if your blood type is AB, for example, you're supposed to be "cool and controlled".
So suppose we decided to invent a new word, "wiggin", and defined this word to mean people with green eyes and black hair—
A green-eyed man with black hair walked into a restaurant.
"Ha," said Danny, watching from a nearby table, "did you see that? A wiggin just walked into the room. Bloody wiggins. Commit all sorts of crimes, they do."
His sister Erda sighed. "You haven't seen him commit any crimes, have you, Danny?"
"Don't need to," Danny said, producing a dictionary. "See, it says right here in the Oxford English Dictionary. 'Wiggin. (1) A person with green eyes and black hair.' He's got green eyes and black hair, he's a wiggin. You're not going to argue with the Oxford English Dictionary, are you? By definition, a green-eyed black-haired person is a wiggin."
"But you called him a wiggin," said Erda. "That's a nasty thing to say about someone you don't even know. You've got no evidence that he puts too much ketchup on his burgers, or that as a kid he used his slingshot to launch baby squirrels."
"But he is a wiggin," Danny said patiently. "He's got green eyes and black hair, right? Just you watch, as soon as his burger arrives, he's reaching for the ketchup."
The human mind passes from observed characteristics to inferred characteristics via the medium of words. In "All humans are mortal, Socrates is a human, therefore Socrates is mortal", the observed characteristics are Socrates's clothes, speech, tool use, and generally human shape; the categorization is "human"; the inferred characteristic is poisonability by hemlock.
Of course there's no hard distinction between "observed characteristics" and "inferred characteristics". If you hear someone speak, they're probably shaped like a human, all else being equal. If you see a human figure in the shadows, then ceteris paribus it can probably speak.
And yet some properties do tend to be more inferred than observed. You're more likely to decide that someone is human, and will therefore burn if exposed to open flame, than carry through the inference the other way around.
If you look in a dictionary for the definition of "human", you're more likely to find characteristics like "intelligence" and "featherless biped"—characteristics that are useful for quickly eyeballing what is and isn't a human—rather than the ten thousand connotations, from vulnerability to hemlock, to overconfidence, that we can infer from someone's being human. Why? Perhaps dictionaries are intended to let you match up labels to similarity groups, and so are designed to quickly isolate clusters in thingspace. Or perhaps the big, distinguishing characteristics are the most salient, and therefore first to pop into a dictionary editor's mind. (I'm not sure how aware dictionary editors are of what they really do.)
But the upshot is that when Danny pulls out his OED to look up "wiggin", he sees listed only the first-glance characteristics that distinguish a wiggin: Green eyes and black hair. The OED doesn't list the many minor connotations that have come to attach to this term, such as criminal proclivities, culinary peculiarities, and some unfortunate childhood activities.
How did those connotations get there in the first place? Maybe there was once a famous wiggin with those properties. Or maybe someone made stuff up at random, and wrote a series of bestselling books about it (The Wiggin, Talking to Wiggins, Raising Your Little Wiggin, Wiggins in the Bedroom). Maybe even the wiggins believe it now, and act accordingly. As soon as you call some people "wiggins", the word will begin acquiring connotations.
But remember the Parable of Hemlock: If we go by the logical class definitions, we can never class Socrates as a "human" until after we observe him to be mortal. Whenever someone pulls a dictionary, they're generally trying to sneak in a connotation, not the actual definition written down in the dictionary.
After all, if the only meaning of the word "wiggin" is "green-eyed black-haired person", then why not just call those people "green-eyed black-haired people"? And if you're wondering whether someone is a ketchup-reacher, why not ask directly, "Is he a ketchup-reacher?" rather than "Is he a wiggin?" (Note substitution of substance for symbol.)
Oh, but arguing the real question would require work. You'd have to actually watch the wiggin to see if he reached for the ketchup. Or maybe see if you can find statistics on how many green-eyed black-haired people actually like ketchup. At any rate, you wouldn't be able to do it sitting in your living room with your eyes closed. And people are lazy. They'd rather argue "by definition", especially since they think "you can define a word any way you like".
But of course the real reason they care whether someone is a "wiggin" is a connotation—a feeling that comes along with the word—that isn't in the definition they claim to use.
Imagine Danny saying, "Look, he's got green eyes and black hair. He's a wiggin! It says so right there in the dictionary!—therefore, he's got black hair. Argue with that, if you can!"
Doesn't have much of a triumphant ring to it, does it? If the real point of the argument actually was contained in the dictionary definition—if the argument genuinely was logically valid—then the argument would feel empty; it would either say nothing new, or beg the question.
It's only the attempt to smuggle in connotations not explicitly listed in the definition, that makes anyone feel they can score a point that way.