An expert is a man who has stopped thinking. Why? He knows.

Frank Lloyd Wright (1867-1959) American architect, interior designer, writer, educator [b. Frank Lincoln Wright]

In Geoffrey T Hellman, “Wright Revisited,” The New Yorker (8 Jun 1956)
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Wright used variations on this quotation throughout his life, e.g.:

The expert is usually a man who has stopped thinking and so is perfectly able to be utterly wrong for at least the rest of his lifetime. He has made up his mind, not upon principle, but upon expedient practice.
[Source, Frank Lloyd Wright: An Autobiography, Book 5 "Form" (1943)]

An expert? Generally a man who has stopped thinking because he knows!
[Source]

An expert is a man who has stopped thinking -- he knows.
[Source, in Earl Nesbit, Taliesin Reflections (2006)]

To me an expert is a man who has stopped thinking. He thinks he knows everything.
[Source, in Patrick J. Meehan, Truth Against the World (1987)]

Now, an "expert" is a man who has stopped thinking. He has had to stop thinking or he would be no expert. You can't call a man an "authority" who is growing and so changing his mind about things, can you? No, the expert has got to know or profess he knows. He's got to stand there and be knowledgeable! Well, too bad, because there's no such human except he be somewhat a phoney.
[Source, in Patrick J. Meehan, Truth Against the World (1987)]

An expert is a man who has stopped thinking because he knows and you can do nothing with him if you got a good idea.
[Source, in Patrick J. Meehan, The Master Architect (1984)