For the record, I've worked in higher education for about 15 years now. I've never heard a professor refer to a less experienced professor as "beginner." They've accumulated a good amount of knowledge and skill just to get to that point in the first place. Which was the point of people upthread, to bring this back to martial arts.
As I said, it all depends on your point of view. Lets say there's somebody who has a doctorate in chemical engineering, they've worked for a bunch of companies as a chemical engineer and now they've decided to teach so they take up a career as a professor in chemical engineering. When they first get started doing that they're not a beginner in chemical engineering but they are a beginner as a professor in chemical engineering.
To get to black belt (or whatever benchmark makes sense for a given style), you have accumulated a notable level of skill. (Again, depending on the school.) You're not a beginner. You're not done, but you're not a beginner.
It depends on what kind of "beginner" you're talking about. You could say a black belt is a beginner of the dan ranks even if they're not a beginner in the martial arts overall, just like a ninth grader is a beginner of high school but not a beginner of school.
So, if we assume that there's always more to learn, no matter what field of endeavour we're discussing, doesn't that essentially make EVERYONE a beginner?
From a certain point of view, yes.
As a matter of fact there are some martial arts instructors who hold high dan ranks who wear a double wide white belt, to symbolize the circular path of learning they've taken.
I knew of this one dojo where they said everybody was a white belt and that a black belt was just a white belt that had been dyed black, no matter what color your belt was on the surface beneath that it was white so everybody was a white belt in that sense.
The art of Jeet Kune Do, the art created by Bruce Lee which doesn't use belts but rather uses patches to denote rank, both the lowest patch and the highest patch is an empty circle, the philosophy is that after you gain all that knowledge, skill, and expertise you reach a new beginning.
I've been involved in martial arts since about 1983, and I've seen plenty of black belts move on. In some sense, I moved on myself (though that was more down to health issues than any sense that I'd finished). That's fine. Nobody signs a lifelong contract. Follow your bliss. But yes, you see a higher attrition rate among lower belts. Because there's more of them.
My experience has been different, I've seen some black belts move on but not that many, twice at my first dojo and once at the dojo Im at now I saw people get a black belt and right after that move on but that's all I've seen of that. The dojos that I've been to, though, you have to earn a black belt, they just don't hand them out. At belt factories where belts are handed out like candy, including the black belt, it might be different. I don't have any experience with those belt factories and I wouldn't waste my time at such places.
From what I've seen many people quit at white belt or after making yellow belt because the martial arts is just something they want to try out, they're just going through a phase to see what its like and they might decide its not for them and move on, or they might stay until they go up one belt and then move on.
That definitely CAN be the effect. It's not universal though.
That's my experience, although for others it might be different.
So which is it? What achievement is it marking if you're still a beginner?
That depends on what you see it as and that of course varies from person to person. As for it meaning that you're a beginner, that also depends on your point of view and what kind of beginner you're talking about, which varies as well.
I'm not arguing that you're done when you get a black belt. I received mine in... 93, I think. It's not like I stopped. I'm simply saying that I don't find "black belt beginner" to be a particularly useful designator. Your mileage may vary.
You're done when you're done and that varies from person to person, a lot. Some people decide they're done when they've just taken a few classes and they find out it's not their thing, but the fact remains that no matter how far you go you can always go further.