Perhaps the single responsibility principle. Before that, it was always hard to decide on a good point between either everything being a single compact block o

Perhaps the single responsibility principle. Before that, it was always hard to decide on a good point between either everything being a single compact block of code doing everything, or the other extreme of everything being separated out to many different abstractions and subsystems just for the sake of it, seemingly. The single responsibility princible gives a good rule of thumb of why and where to split things up, and if you follow it, you will by design get a system which has about enough abstractions for change to be reasonably easy.