Where does this quote appear in the book Into the Wild? "If we admit that human life can be ruled by reason, then all possibility of life is destroyed."
I cannot find this quote in the book Into the Wild, but since it comes from War and Peace by Leo Tolstoy, it seems that it would be appropriately associated with Chris McCandless, who was an avid fan and reader of Tolstoy. My guess is that the quote occurs in the film, which I have not seen. The quote itself refers to the passion of living for self and others, and not being ruled strictly by "reason," which in Tolstoy's definition meant the utilitarian approach, living without empathy and striving only for one's self. In other words, while reason is important in not committing stupid mistakes, life cannot exist without altruism and empathy. Chris would have agreed with those sentiments, and in fact he highlighted a quote from Tolstoy's Family Happiness which he felt described his feelings:
Satisfied, apparently, with what he had learned during his two months of solitary life in the wild, McCandless decided to return to civilization...
...work which one hopes may be of some use; then rest, nature, books, music, love for one’s neighbor -- such is my idea of happiness.
(Krakauer, Into the Wild, amazon.com -- Tolstoy, Family Happiness, quoted)
The quote on reason, then, is one that would have resonated with Chris and added to his personal philosophies. Chris did not allow himself to be ruled by reason, and so lived his life deeply in the moment, able to enjoy himself and others without feeling the need for a rational "give-and-take" relationship. Chris instead lived without expectations, although a lack of reason is what led to his death.