Earlier quoted context omitted.
It's obviously not true. Teaching is a distinct skill (it's probably a large basket of distinct skills). People having varying aptitudes for it. Common sense tells you that the cliche can't possibly be true. George Bernard Shaw put those words deliberately into the mouth of a character you in particular would find absolutely insufferable.
To go even further, I find a saying attributed to Aristotle to be quite true: "Those who know, do. Those that understand, teach." I find that true experts are able to explain subject matter in their domain simply , they can ably distill complicated issues to their essence lest their pupil be bogged down in unnecessary side-details. I used to think Raymond Hettinger was just an awesome teacher because he could make ce…
But I agree, it's cringe-y to see people try to dunk on teachers like this. There are good teachers and bad teachers like there are good and bad everythings; if you've had a good teacher for something before, it's hard for me to imagine that you'd take Shaw's character seriously with the "those who can" stuff.