Section 4 of the 25th Amendment, ratified in 1967, establishes the only constitutional mechanism for involuntary presidential removal that does not require impeachment. It requires the vice president and a majority of the principal officers of the executive departments — the 15 Cabinet secretaries — to send a written declaration to the Speaker of the House and the Senate president pro tempore stating that the president "is unable to discharge the powers and duties of his office." The vice president then immediately assumes the powers of acting president. If the president contests the declaration in writing, Congress must convene within 48 hours. Congress then has 21 days to vote: if two-thirds of both the House and Senate agree that the president is unable to serve, the vice president continues as acting president. If Congress does not reach that threshold, or if it fails to vote, the president resumes full powers. Section 4 has never been invoked in U.S. history. Legal scholars disagree about whether it covers voluntary cognitive incapacity only, or whether it could apply to behavioral fitness. Its political barriers are substantial: the vice president and Cabinet who must initiate it serve at the president's pleasure.