The other day I was taking an introductory training class for some technology at work. There was a slide that outlined the technology, and one of the bullet points had an asterisk next to it. At the bottom of the page was this footnote:
Most strong statements like this are only mostly true. Don’t worry about it.
I had to stop for a while to ponder the pedagogical implications of this footnote.
There's an inherent problem in trying to describe something complicated to a newbie: how do you start? If someone knows absolutely nothing about, say, playing bridge, or verbs in Spanish, or physics, or grammar, you have to give them a large-picture, broad-stroke overview of this thing they're about to dive into.
This is hard. One reason is that people who are familiar with some domain frequently have difficulty coming up with sufficiently high-level overviews that make sense to a beginner. I've had a couple of people attempt to explain the game of bridge to me, but they could not come up with a simple, comprehensible explanation of the bidding process.
A closely related reason is that experts often cannot let go of details. For example, in your first week of Spanish class, the teacher tells you that the verb hablar means "to speak," and that to say "I speak" you cut off -ar and add -o: hablo. And that this is the pattern for any verb that ends in -ar. So to say "I take," you use the verb tomar and turn it into tomo.
Easy! Powerful! Also, of course, only mostly true: there are irregular verbs and reflexive verbs and other fun. But throwing those additional details at you in the first week of Spanish 101 is counterproductive. There will be time to sort out the exceptions later, once you understand some basics.
I took physics in high school, and when you start, you're learning a lot about
f=ma. I have memories of homework problems involving blocks being pulled or pushed, and the problems always said something like "… ignoring the effects of air resistance." A beginning physics student has enough to think about when calculating the effect of gravitational acceleration without trying to factor in air resistance and all the other real-life variables that come into play. In fact, there's a well-known joke in the physics community about a "spherical cow" that represents the ultimate in simplifying a model.
One more example. In the linguistics community, it's widely discussed that even if kids are taught grammar, it's not taught very well. People who are experts in grammar will sometimes complain (example) that the explanations we give students are hopelessly simplistic. "A noun is the name for a person, place, or thing," goes a typical definition. This doesn't adequately cover gerunds ("Smoking is bad for you") or concepts ("Orange is the new black") or many other ways in which we noun things.
But this gets back to the point. If you're faced with a classroom of 8-year-olds, how do you tell them what a noun is? Using terms like "lexical category" and "defined by its role in the sentence" is not going to work. You have to start somewhere.
And that means ignoring messy details. As one of the commenters on the linked grammar post describes it, "It's quite normal for us to use 'lies to children' in education." Or, to get back to where we started, you sometimes have to make strong statements that are only mostly true.