It is urgent for Americans to think and speak clearly about President Trump’s inability to do either. This seems to be not a mere disinclination but a disability. It is not merely the result of intellectual sloth but of an untrained mind bereft of information and married to stratospheric self-confidence. In February, acknowledging Black History Month, Trump said that “Frederick Douglass is an example of somebody who’s done an amazing job and is getting recognized more and more, I notice.” Because Trump is syntactically challenged, it was possible and tempting to see this not as a historical howler about a...