Harry Potter and the Prisoner of Azkaban
"And how do you conjure it?"
"With an incantation, which will work only if you are concentrating, with all your might, on a single, very happy memory." (12.3.11-12)
Terrible though it was to hear his parents' last moments replayed inside his head, these were the only times Harry had heard their voices since he was a very small child. But he'd never be able to produce a proper Patronus if he half wanted to hear his parents again [...] (12.3.76)
"Oh no," said Lupin. "Much worse than that. You can exist without your soul, you know, as long as your brain and heart are still working. But you'll have no sense of self anymore, no memory, no [...] anything. There's no chance at all of recovery. You'll just – exist. As an empty shell." (12.5.22)