This is a follow-up to this question:
I asked, when a composer has indicated in the score that the performer play softly (eg. pp), how does the composer then indicate the performer should go back playing "normally".
The answer kindly given by darren-ringer is that there is no such thing as "normal".
This doesn't sit right with me. If "mp" means "slightly softer", and "mf" means slightly louder, then there must be a "canonical" level, a level that is neither soft nor loud. The level that mp is slightly softer than, and that mf is slightly louder than.
I am by no means an expert musician. I've just always assumed there is a "zero" level.
Is Darren right? Is there no such thing as "normal"? If there is, what is this zero-level called, and how would a composer denote it in a score?