From Richard Cohen at the WaPo:

Under the 25th Amendment to the Constitution, the vice president, together with a “majority of either the principal officers of the executive departments or of such other body as Congress may by law provide” can remove the president for being “unable to discharge the powers and duties of his office.” No doubt the mere mention of incapacitation would summon a horde of lawyers to Washington to contest it or the meaning of every term.

From Eric Zuesse:

Trump, by choosing an Establishment V.P. and an Establishment Cabinet, has virtually invited an Establishment coup, unless he buckles early to the Establishment and violates every progressive promise he had uttered during his campaign for the Presidency.

From Jazz Shaw:

The Vice President, along with a majority of cabinet members, can meet and vote to declare him unable to serve. They then send a letter to the President pro tempore of the Senate declaring their decision and the Veep takes over the duties. But as soon as the President feels he is ready to resume his service, he can transmit a corresponding letter and take the office back without asking anyone’s permission.

The sole exception provided in the fourth clause is the hypothetical case where the Vice President disagrees that the President is capable of dispatching his duties and – again, along with a majority of the cabinet – sends a contradicting letter stating his objection. Congress then has to vote on the matter to decide.