Since Donald Trump has been diagnosed with COVID-19, the health of the president is suddenly an issue. And given the nasty trajectory the disease is capable of taking, the possibility of presidential disability must be considered. Unsurprisingly, Speaker of the House Nancy Pelosi has taken the opportunity to score political points by considering legislation to change the specific procedures on how to deal with presidential disability — a move that is sure to go nowhere but no doubt thrills her party’s liberal base. Political angling aside, there are some basic questions members of the public are surely wondering. What happens if the president is incapable of discharging his duties? What do law and history tell us? While it is unlikely that these will actually come into play, it’s no doubt possible, and understanding what our government is supposed to do can be useful. The 25th Amendment, ratified in 1967, deals with vacancies in the executive office. It explicitly affirms that if the presidential office is vacated, the vice president becomes president. This had been understood by implication and historical tradition, but when President William Henry Harrison died in 1841, it was not entirely clear whether Vice President John Tyler became president in his own right or simply acting president. Tyler importantly asserted that he was the president — arguably his only positive contribution to American history (he would later vote to secede from the Union...