I think the difference there is that Hufflepuff is a voice representing some of Harry’s thoughts and attitudes, running in parallel with the voice that Harry thinks of as himself. His dark side is a different state of consciousness—while it’s “on”, Harry processes emotions differently to normal, as well as thoughts, and the difference is big enough for him to perceive a separation between the dark side and his usual self.
Anecdotal evidence: I have a mental separation in my own inner life between “modules”, things like Harry’s house avatars that interject thoughts into my ongoing thought process, and “other personalities,” only one of which can be running at once. So the structure isn’t totally unrealistic.
Harry says it is, in Azkaban.
I think the difference there is that Hufflepuff is a voice representing some of Harry’s thoughts and attitudes, running in parallel with the voice that Harry thinks of as himself. His dark side is a different state of consciousness—while it’s “on”, Harry processes emotions differently to normal, as well as thoughts, and the difference is big enough for him to perceive a separation between the dark side and his usual self.
Anecdotal evidence: I have a mental separation in my own inner life between “modules”, things like Harry’s house avatars that interject thoughts into my ongoing thought process, and “other personalities,” only one of which can be running at once. So the structure isn’t totally unrealistic.