An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field.

Niels Bohr (1885-1962) Danish physicist

(Attributed)

As quoted by Edward Teller, in "Dr. Edward Teller's Magnificent Obsession" by Robert Coughlan, LIFE magazine (6 Sep 1954). Variant: "An expert is a man who has made all the mistakes which can be made in a very narrow field."