I'm sure he would say he isn't stupid, but that's part of the problem. No one is intelligent in all domains (contrary to the article, I'm afraid); Trump's intelligence lies in speaking convincingly to a portion of the population, which at least in November 2016 and November 2024 constituted a majority of voters who actually voted. But when it comes to economics (consider: tariffs, but also inflation and the need for scientific research to sustain our economy), science (especially medicine, in view of his choice of Lysenko Jr), world politics (doing things which push former allies away, sometimes even in the direction of China), negotiating (he's like an amateur chess player who sees a chance to take his opponent's pawn without noticing that this allows the opponent to take his rook), psychology (complete befuddlement when dealing with people like Putin, Xi and the Dear Leader), law (seeing solid cases against his personal enemies where there is no evidence, much less a real case), and...
I could go on. My point is that Trump is really only smart in one area, which he thinks makes him smart in all areas. He isn't: he's dumb in those other domains.
