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A justice department opinion on how the 25th amendment works
https://www.justice.gov/sites/default/files/olc/opinions/1985/06/31/op-olc-v009-p0065_0.pdf
The first question is, who are the “principal officers of the executive departments” who must participate with the Vice President in the determination of disability?
We believe that the “principal officers of the executive departments” for the purposes of the Twenty-Fifth Amendment are the heads of the departments listed in 5 U.S.C. § 101, presently the Secretary of State, Secretary of the
Treasury, Secretary of Defense, Attorney General, Secretary of the Interior, Secretary of Agriculture, Secretary of Commerce, Secretary of Labor, Secretary of Health and Human Services, Secretary of Housing and Urban Development, Secretary of Transportation, Secretary of Energy, and Secretary of Education. This view is supported by the legislative history of the Amendment.
Furthermore, although the acting heads of departments and recess appointees are not Presidential successors, see 3 U.S.C. § 19(e), the legislative history of the TwentyFifth .Amendment suggests that, in the event of a vacancy in office or the absence or disability of a department head, the acting department head, at least at the level of undersecretary, principal deputy, or recess appointee might be entitled to participate in determinations of Presidential disability