Has it been used before?

Since its adoption in 1967, the first three sections of the 25th Amendment have been used several times.

The first and second sections were used in 1974, when Richard M. Nixon resigned the presidency and was replaced by Gerald Ford, his vice president. Mr. Ford subsequently nominated Nelson Rockefeller to be vice president, and Mr. Rockefeller was confirmed by the House and the Senate.

Section 3, which allows presidents to temporarily shift their powers and duties to the vice president, was used by Ronald Reagan in 1985 when he underwent a brief cancer surgery. President George W. Bush also invoked the 25th Amendment in 2002 when he underwent a brief medical procedure and transferred his duties to Vice President Dick Cheney for a few hours. Mr. Bush did the same thing again in 2007.

The fourth section of the 25th Amendment — the one contemplated by the anonymous author of the essay in The Times — has never been used.

How would it actually work, if invoked now?

The first step would be for Vice President Mike Pence and a majority of the cabinet to provide a written declaration to the president pro tempore of the Senate (currently Senator Orrin G. Hatch of Utah) and the speaker of the House (currently Representative Paul D. Ryan of Wisconsin) that Mr. Trump “is unable to discharge the powers and duties of his office.” That would immediately strip Mr. Trump of the powers of his office and make Mr. Pence the acting president.

But the 25th Amendment would allow Mr. Trump to immediately send a written declaration of his own to Mr. Hatch and Mr. Ryan saying that he is in fact able to perform his duties. That would immediately allow him to resume his duties, unless Mr. Pence and the cabinet send another declaration to the congressional leaders within four days restating their concerns. Mr. Pence would take over again as acting president.

That declaration would require Congress to assemble within 48 hours and to vote within 21 days. If two-thirds of members of both the House and the Senate agreed that Mr. Trump was unable to continue as president, he would be stripped permanently of the position, and Mr. Pence would become president. If the vote in Congress fell short, Mr. Trump would resume his duties.