"I am not an expert on Brecht," said the professor. "I haven't read even a quarter of what's been written about him."
Now, if you were to ask me, I would suggest that an expert on Brecht would have thorough knowledge of (a) Brecht's works, (b) his style, (c) literary, social, and personal influences in Brecht's life, and (d) Brecht's contributions to his art and to society. But then I've always been mildly countercultural.
In modern "western" academia, to be an expert about a person is to be familiar with what otehr people say about that person. The expert to academics is one who knows nearly everything that has been written about the subject.
And this makes no good sense. What other people write about Brecht may indeed help one to understand Brecht's works and influence; what other people say can have value. But nearly everyone who has written about Brecht has less stature than Brecht himself. How does knowing the minds of second and third tier intellects build expertise about the original? How does hearsay strengthen a case? This is a recipe for creating fourth and fifth tier intellects (and perhaps for walling out the best and most creative minds).
No; knowledge about what others say can be useful but expertise is a term which should only be reserved to knowledge of the original.