Mr. Granger: In Deathly Hallows, Ms. Rowling writes that Harry witnessed his mother’s death. If this is true, why doesn’t he see the thestrals until after Cedric dies?
I believe Ms. Rowling has answered this in an interview or her web site; please do a search at JKRowling.com and Accio Quotes!
My quick answer, probably reflecting what I’ve read with my own thoughts stirred in, is that the ability to see Thestrals, a transformed vision in a Saga largely about seeing correctly and at greater depth, is a function of how the mind changes in its ability to grasp profound and difficult meaning — represented neatly by the mythic creatures suggestive of death, the Thestrals — after a confrontation with life’s ending.
Harry experience of death as an infant was not changed by what he saw, at least not internally, hence it is Cedric’s death in the Hangleton graveyard which opens his eyes, if you will. Great question — and please let me know what you find in your search for Ms. Rowling’s answer. [For more on the eyes of Deathly Hallows and Coleridgean transformed vision in imaginative literature, see chapter 5 of The Deathly Hallows Lectures, ‘The Seeing Eye.’]

David C. Downing
Recent Conversation