The American Presidential succession process is interesting in that it starts off in the executive branch (with the VP), moves to the legislative branch (Speaker of the House, then President Pro Tem), then moves back to the executive to each cabinet officer from oldest to most recent creation of the department. In looking at this state of affairs, I learned that President Truman wanted it this way:
Insofar as possible, the office of the President should be filled by an elective officer. There is no officer in our system of government, besides the President and Vice President, who has been elected by all the voters of the country. The Speaker of the House of Representatives, who is elected in his own district, is also elected to be the presiding officer of the House by a vote of all the Representatives of all the people of the country. As a result, I believe that the Speaker is the official in the Federal Government, whose selection next to that of the President and Vice President, can be most accurately said to stem from the people themselves.
And Truman's wishes were enshrined in the Presidential Succession Act of 1947. The 25th Amendment / Amendment XXV revisits some of these issues, like enshrining that the Vice President must be the first in order of succession, creating the process of filling a vacant office of Vice President, and mandating procedures for handing presidential incapacity other than death, and was ratified in 1967 while Truman was still alive.
What did he think about the 25th Amendment? What are sources that provide further details on Truman's thoughts on the 25 Amendment?