Ben Carson was a (by all accounts excellent) brain surgeon.
I'm sorry, but that man is stupid.
Brains are weird, man. I work in a STEM field, but I had 3 or 4 semesters of University before declaring my major, and therefore I was able to get a much more well-rounded education than my colleagues, and I will tell you: It shows. Big time.
Lots of people who are great at what they do, and when it comes to their one very specific, silo'd, expertise, they're brilliant.
But in terms of general intelligence, rationality, ability to think critically in a novel situation, etc? Not bright.
Then there's the old (true) joke: What do you call someone who graduated at the bottom of their class in medical school? Doctor.