Δαφνη Ευτυχια Αρθουρος

"Those who can, do. Those who understand, teach." -- Lee S. Shulman, "Those who understand: Knowledge growth in teaching", Educational Researcher vol. 15, no. 2, February 1986 (pp. 4-14)

For folks who perfer a snippet longer than a bumper sticker ... the author begins by quoting (and then complaining about) George Bernard Shaw's famous line that "He who can, does. He who cannot, teaches." Somewhere in the middle of the paper (p. 7) he refers to Aristotle:

Aristotle, whose works formed the heart of the medieval curriculum, made these observations in Metaphysics (cited in Wheelwright, 1951):

"We regard master-craftsmen as superior not merely because they have a grasp of theory and know the reasons for acting as they do. Broadly speaking, what distinguishes the man who knows from the ignorant man is an ability to teach, and this is why we hold that art and not experience has the character of genuine knowledge (episteme)--namely, that artists can teach and others (i.e., those who have not acquired an art by study but have merely picked up some skill empirically) cannot."

His closing (p. 14) is a callback to both of those earlier references:

We reject Mr. Shaw and his calumny. With Aristotle we declare that the ultimate test of understanding rests on the ability to transform one's knowledge into teaching.

Those who can, do. Those who understand, teach.

Wikiquote notes that the short form is often attributed to Aristotle -- and that's how I first saw it, but it didn't sound right, so I went to Qikiquote to check, and found there a helpful link to the paper it's from.