On which page of Into the Wild does the quote about "the core of man's spirit" appear?
That quote appears in chapter six of Krakauer's Into the Wild. The full quote actually reads as follows: "The very basic core of a man’s living spirit is his passion for adventure."
I can't tell you the exact page number, because I am looking at a PDF version, and different text editions will be slightly different in their page numbering. The quote is on page 41 of the document that I am looking at, but I can give you better surrounding information to help you locate it.
The quote is closer to the beginning of chapter seven than it is to the beginning of chapter six. The quote itself is found in the long letter than McCandless wrote to Ronald Franz. In the letter, McCandless speaks very passionately about why Franz should make a "radical" change to his life. He wants Franz to stop staying where he is and take to the road in order to have "an endlessly changing horizon."
The joy of life comes from our encounters with new experiences, and hence there is no greater joy than to have an endlessly changing horizon, for each day to have a new and different sun. If you want to get more out of life, Ron, you must lose your inclination for monotonous security and adopt a helter-skelter style of life that will at first appear to you to be crazy. But once you become accustomed to such a life you will see its full meaning and its incredible beauty. And so, Ron, in short, get out of Salton City and hit the Road. I guarantee you will be very glad you did.