"The final version of the 25th Amendment provides that when the Vice President and a majority of the Cabinet determine that the President is unable to perform his duties, the Vice President becomes the acting President," the article details. That determination is thus reviewed by Congress.
A beleaguered President may attempt to rebut this assertion, but Congress has the final say, and that say must be in the form of a two-thirds vote to remove from office.
This is also an interesting detail: "there’s no particular threshold that needs to be reached before the relevant body -- in this case the Cabinet -- starts thinking about" evoking the 25th Amendment.
When a President's sanity is in question, the process is both political and scientific. Trump clearly appears to be mentally ill, but how ill must he be before the American people put enough pressure on the Cabinet and Congress to act?
Hopefully we'll never reach that point where the President has tumbled into madness. Better to remove Trump first by impeachment.
Nevertheless, he is arguably the most dangerous President in American history due to his serious mental instability, not to mention his complete corruption. Things cannot continue as they are now for too long.
The most amusing -- or perhaps not -- aspect of this discussion is that Trump reportedly had no idea, until the subject was raised, what the 25th Amendment was. "What’s that?" when the amendment was discussed.