Hope. "Man can live about forty days without food, about three days without water, about eight minutes without air, but only for one second without hope." (Hal Lindsey)
"Take from a man his wealth, and you hinder him; take from him his purpose, and you slow him down. But take from man his hope, and you stop him. He can go on without wealth, and even without purpose, for a while. But he will not go on without hope," is what C. Neil Strait said.
Film actor Christopher Reeve said: "Once you choose hope, anything's possible."
Hope Quotes here: http://www.great-inspirational-quotes.com/hope-quotes.html