At the Constitutional Convention in 1787, Delegate John Dickinson of Delaware raised two questions that, in Madison's notes at least, met with an uncomfortable silence: "What is the extent of the term 'disability' & who is to be the judge of it?" Dickinson was referring to what became the Presidential Succession Clause in Article II. Debate on this provision was immediately postponed, and Dickinson's prescient questions went unanswered for almost two centuries. In 1967, a new generation of constitutional authors answered his second question in the Twenty-Fifth Amendment (the "Amendment"), which gives two sets of constitutional actors the power to declare presidential inability. Section 3 makes the President judge of his own inability:
Whenever the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, such powers and duties shall be discharged by the Vice President as Acting President.
Adam R. Gustafson,
Presidential Inability and Subjective Meaning,
Yale L. & Pol'y Rev.
Available at: https://digitalcommons.law.yale.edu/ylpr/vol27/iss2/7