I've always thought about the relationship between key signature note
s, and accidentals
rather like the philosophical distinction
The former--the necessary--is part of the fabric of the universe, and must occur; the latter--the accidental--is not part of the fabric of the universe, and is just an accident.
This doesn't really hold up too well, but I have always found it amusing. And I rarely share it with my students, as they are with me for music, not philosophy--or humor.