Something I see coming up a lot in discussions of the Republican field of candidates is mystification over how Ben Carson can keep saying so many dumb things. "But he's a brain surgeon!! You need to be smart for that!!1!!1!!1!one!"
What people are missing on this is something my undergraduates have trouble with all the time: knowledge in one area, even very advanced knowledge, does not give one any knowledge in any other field. Sure, it takes a fair bit of smarts to be a brain surgeon (though in many ways it's as much about working hard as it is being smart, but that's another post for another day). But nothing about brain surgery teaches you about American history, or about the tenets of democracy, or the about the constitutionality of various ideas, etc.
As I always take care to explain to my students the first week of every class I teach, not only should you always check to see if any given source is credible, but if it's credible on that specific issue. The example I always use is of myself and an Ear, Nose, and Throat (ENT) physician. Both of us are doctors, but we're very different kind of doctors with very different kinds of expertise. So even though I'm a doctor, if you came to me for advise on your strep throat, I would be pretty useless. Just like if you went to the ENT and asked about best practices in post-conflict police reconstruction.
Both I and the ENT are people who are experts about one thing. That expertise is non-transferrable -- sure, this hypothetical ENT or myself may have some good knowledge of other subjects, but that knowledge is completely coincidental to our credentials.
So why does Ban Carson keep saying such stupid shit constantly? Because electoral politics is literally not brain surgery. He's shown that he's quite adept at the latter, but fucking terrible at the former. And that's not confusing, it's just a demonstration of the basic fact that one highly specific set of knowledge does not guarantee one has even basic knowledge of any other subject.