Two of the giants of quantum mechanics this week, making almost — but not quite — the same point about expertise. (Please excuse the sexist language in both.) First, Werner Heisenberg:
An expert is someone who knows some of the worst mistakes that can be made in his subject, and how to avoid them.
W. Heisenberg, Physics and Beyond (1971)
And now his mentor Niels Bohr’s take on the matter:
An expert is a man who has made all the mistakes which can be made — in a very narrow field.
(Quoted in A. L. Mackay’s The Harvest of a Quiet Eye, 1977.)
Personally I suspect that Bohr’s definition is the more accurate.