Even with his "plighted wife" Elizabeth, the minister refuses to remove the veil. Elizabeth thinks that the veil might well be a "symptom of mental disease." What is the specific nature of the disease? What evidence is there that she might be right?
The specific nature of the disease is probably that he committed a sin or many sins that he cannot forgive himself. The origin of it is unknown, but it seems that something has been bothering him for a long time. He is the reverend, so he must have heard a lot about others. The things he heard may be sins, so he wears the veil to 'hide' the sins, since only God and the dead people can see past his veil.
Some evidence that Elizabeth might be right are when Mr. Hooper refuses to take off the veil, not even for his wife, and that he spoke mysterious words that has somewhat a hidden meaning for others to know.
Hawthorne writes: "All through life that piece of crape and hung between him and the world; it had separated him from cheerful brotherhood and woman's love, and kept him in that saddest of all prisons, his own heart." What does the veil symbolize?
The veil symbolizes Mr. Hooper's sins. Mr. Hooper wears the veil to hide not only his face, but his sins too, so other people cannot see them. He is a minister too, so the veil can motivate him to become a better minister and to be more honest. His wife says that Mr. Hooper's words are mysterious, but only God is truly mysterious, so God and the dead people can see past Mr. Hooper's veil.
Mr. Hooper seems to feel a bit guilty in that passage in the question, since he is isolated in some ways among other people and even himself, his heart. As I've said above, his own sins can be too much for himself, so he wants to block it all away from his life on Earth. Since he died with the veil on his face, his sins may be stuck on Earth or
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