I’ve been thinking about the concept of common knowledge. The topic was thrust upon me by three events.

On Monday, as I tidied up the store, I did the usual Monday moving of the books about Mexico. I have mentioned my colleague JJ before. She works on Saturdays and I usually don’t, so I haven’t worked with her in some time, but we have a little unspoken interaction every week. She moves the books on Mexico to the South America shelf on Saturdays, and on Mondays I move them back to North America. I once mentioned, in the course of a contrived conversation, that Mexico was actually in North America and wasn’t that surprising, but she persists in thinking that Mexico is in South America, and so we move the books back and forth.

On Tuesday, in my evening class, we were discussing the difference between “attain” and “obtain.” It was in the homework, so we had thought about this ahead of time, and someone asked me what dictionary I used. “Oh, I don’t use a dictionary,” I said. My classmates stared at me, and I thought of my unabridged dictionary at home, which I love. “Well, I clarified, “not for words like that.” The staring continued, as it was borne in upon me that for these intelligent women, “attain” and “obtain” were not ordinary everyday words that no one would bother using a dictionary for.

“I’m feeling really bad about this,” I mumbled, “so maybe we could move on.”
“Are you running for President?” asked a soft-spoken lady to my right.

Then yesterday evening a customer asked for a poster on mathematical terms. “Like there’s the addend and the sum,” she said, “but what do you call it for subtraction?”

That Man had just arrived, and he is an accountant, so I deferred to him on that question. He came up with “subtracand,” which sounds like a mysterious far-off land, but that didn’t seem quite right. I sprang to the computer and googled it, and sure enough, it wasn’t right. Our next thought was “subtractand,” which turns out to mean something altogether different. The answer, it turns out, is “subtrahend” for the thing you subtract and “minuend” for the bit you subtract it from. There were cries of “Of course!” because that is something that at least some of us had learned at some point.

Now, the thing about Mexico is understandable to me. When JJ and I were kids, we were never taught that Mexico was part of North America. North America consisted of Canada and the U.S. Mexico was in Central America or Latin America, and Greenland was never even mentioned. The solar system changed its order since then, too, and then its number of planets. The number and content of the food groups has changed. The Periodic Table of the Elements is different. Things have been proven and disproven. The names and borders of countries are different. So a lot of common knowledge, if you are old enough, has changed.

The thing about attaining and obtaining also makes sense. I find words interesting, and think about them. There are lots of things on which I am woefully ignorant. I was, apparently, the last person on the planet to find out about Paris Hilton’s underpants. We tend to think that things we find interesting are common knowledge, things everyone should know, while things we don’t care about are arcane. The comment about presidents, after all, relied on our all knowing that Biden had put his foot in his mouth while kicking off his presidential campaign. Apolitical people might not know that.

And the subtrahend issue also makes sense. We may have learned that sometime in the past, but it doesn’t come up in conversation often enough for us to keep it in the forefront of our minds. Plenty of things fall into the category of stuff that is common knowledge, but most of us would still have to look it up.

And yet we continue to behave as though there were a pool of common knowledge that we all ought to have. Otherwise, of course, we couldn’t talk to one another at all. But we draw conclusions about people based on our assumptions about common knowledge. I remember, when I was a grad student, one of the profs coming in with a sigh of disgust. He had given his students a test of general knowledge and they didn’t know anything! He gave a couple of examples: they didn’t know who Paul Newman and Gypsy Rose Lee were!

Paul Newman, boys and girls, was a movie star a long time ago, and Gypsy Rose Lee was a stripper. So this guy was basing his judgement of the ignorance of his students on their knowledge of the celebrities of his youth. In fifty years, will you consider people well-informed only if they know who Paris Hilton was?

There is a position in the education community that says that there is no longer any point in teaching facts. There is so much information, this point of view says, that the only thing worth teaching is how to look things up. I can’t agree with that. I think there still is a shared body of common knowledge, or at least that there ought to be. We argue about this a lot in education circles, in the context of what should be taught.

But in daily life, courtesy demands some decision about whether or not a thing is common knowledge. It is not courteous to use words like “subtrahend” to people who don’t know them. I don’t use “rodomontade” in daily conversation, myself, though I would not have had any qualms about “attain.” It isn’t courteous to correct people who think Mexico is in South America — though obviously I can’t leave the books in the wrong section. It’s bad enough that our Saturday shoppers think we don’t know any better than that.

Our assumptions about what is common knowledge may actually divide us. We are more relaxed in speaking with people when we do not have to worry that a word choice or an allusion will be missed — or, worse, seem to be intended to make the hearer feel lessened. People who know for sure what SIMMs and DIMMs are end up talking to one another, and those who are clear on SKPssos and K2togs talk to one another.

Speaking of K2togs, Pipes is down to one sleeve’s ribbing and the collar. #2 daughter is coming down next weekend for an optometrist’s appointment, and I hope to have it finished by then.

Although it is possible that she will not be down next weekend. I hear from #1 son, who saw it on Facebook, that someone called the eye doctor and mentioned that he needed contact solution. I had done just that, but I spoke to the optometrist herself, who was not the one who mentioned it on Facebook. Still, I had mentioned all this to #2 daughter via IM that morning, so she may have called back later to change the appointment — there was concern, apparently, that Chinese New Year was a bad time for her to be away from the office — or, indeed the optometrist might have told the person who posted it on Facebook.

At the very least, we can be assured that modern technology gives us the opportunity to turn rumor into common knowledge faster than ever before.

I have come back here, even though I should be starting those huevos rancheros for the boys, because I simply must share some cool knowledge via the internet. If you click right here, you will find free Burda sewing patterns to print out.