> a simple question: "What keeps a human alive?"

I was spending some time reading a book and found a simple but 'deep' question: "What keeps a human alive?" (Aleksandr Solzhenitsyn - "Cancer Ward") It is a simple question and most of us (if not all of us) had it at least once in his/her lifetime and, most likely, he/she also tried to find an answer (It is not necessary to reach this question by reading it in a book - and also not necessary to find the answer in a book). Take a dare...!?