Every day, it seems, the president of the United States wanders off during a photo shoot or tries to shake hands with an imaginary person or garbles his way through another speech, and the rest of us are expected to go on with our day as if everything is just fine. It’s not fine. He’s not fine. And, no, this isn’t about Joe Biden’s longtime propensity to lose his temper or his alleged lifelong stutter or his habit of inventing fairy tales about his past. It’s about the doddering and scatterbrained behavior that’s familiar to anyone who’s ever dealt with...