"Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President."
I'm sure mental instability is a valid reason for the Veep and either a majority of the Cabinet or the House to invoke this clause.
Let's use just one example of how President Trump sees reality.
This is what he says happened:
Here's the video:
Compare the two, and see if you don't think the President is suffering from a severe form of self-delusion or cognitive dissonance.