What is the reason John is on his journey in "By the Waters of Babylon"?
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At the beginning of the story, John informs the reader that his father is a priest, and he is going to become one too. When John finally becomes a man, he submits to purification in the Priest House as part of the process to becoming a priest; another part of the process involves going on a journey. John looks into the purifying fire and sees "a river, and, beyond it, a great Dead Place and in it the gods walking." His father warns him that this is a strong dream, and it "may eat him up."
John goes on his journey because he believes in the power of his prophetic dream. Even though his father has told him that to go to the East is forbidden, John looks for signs, like the panther, to affirm that is where he must go.