I've seen people hung up on the calling yourself while defining yourself concept. Talking through that often highlighted a mental model of functions where they were sort of almost cut'n'pasting the other function into their code. Basically their model was too low level and the understanding of defining and calling functions were unnecessarily conjoined.
I guess the way I’d try to help those people understand it is with the fiction I tell myself (which I alluded to upthread): I mentally model the “outer” function and the “inner” recursion as referencing different functions that have the same signature and the same body. The definition is concrete and self sufficient, it just dependency-injects a contract in its own image to delegate future work to. It’s even easy to literally implement that way in languages with first class functions, because you can just bind fn a to b and call b within a.
