An expert is someone who knows some of the worst mistakes that can be made in his subject, and how to avoid them.

Der Teil und das Ganze. Gespräche im Umkreis der Atomphysik (1969); also in "Kein Chaos, aus dem nicht wieder Ordnung würde", Die Zeit No. 34 (22 August 1969); as translated in Physics and Beyond : Encounters and Conversation (1971)