During Mr. Bayh’s courtesy call on the House speaker, John McCormack, the representative complained that “one of those so-called lady reporters asked me if I felt I was qualified to serve as president.” As a legislator and World War I veteran, Mr. McCormack had clocked almost a half-century as a public servant — and still some persnickety dame raised on promises presumed to question whether he was presidential material. The nerve.

“Landslide” by Jane Mayer and Doyle McManus is, like Mr. Bayh’s book, out of print but well worth tracking down through used-book sellers and libraries. It is a thoroughly reported and riveting account of Ronald Reagan’s second term. A harrowing prologue opens with Jim Cannon, a 69-year-old bureaucrat in pajamas, scanning his bookshelves for his copy of the Constitution and flipping to the 25th Amendment. It was March 1987, and President Reagan’s new chief of staff, Howard Baker, had given Cannon and a colleague the task of interviewing the White House staff to understand its Iran-contra-era disarray. Various aides shocked Cannon with stories “about how inattentive and inept the president was. He was lazy; he wasn’t interested in the job. They said he wouldn’t read the papers they gave him — even short position papers and documents. They said he wouldn’t come over to work — all he wanted to do was to watch movies and television at the residence.”

Presented with these findings, Baker, along with Cannon and other aides, scrutinized Reagan to assess his mental fitness. The president was joking around, having a good day. He impressed his monitors as “alert and attentive” and, as worded in Section 4 of the 25th Amendment, able “to discharge the powers and duties of his office.” Mr. McManus and Ms. Mayer’s reporting unearthed other senior moments such as the one in which a Reagan crony told them about the president’s forgetting “who he was talking to in the midst of a phone conversation.” Their book was published in 1988, six years before the president disclosed his Alzheimer’s diagnosis to the nation.

The subset of lies told by the current scatterbrained, TV-addicted occupant of the Oval Office — descriptions of phone calls with the Mexican president and Boy Scout officials that never took place, a made-up terrorist attack in Sweden that prompted a former Swedish prime minister to ask, “What has he been smoking?” — has my inner Jim Cannon repeatedly reaching for my 95-cent copy of the Constitution and paging to Amendment XXV.

Are such falsehoods mere fibs? Or do they bring to light an inability to distinguish fantasy from reality? Which, incidentally, happens to be the legal definition of insanity.

Grilled at a news conference in 1958 about the constitutionality of his memo to Nixon, Eisenhower said: “I admit this: If a man were so deranged that he thought he was able, and the consensus was that he couldn’t, there would have to be something else done, no question.”