In the late nineteenth century Unamuno suffered a religious crisis and left the positivist philosophy. Then, in the early twentieth century, he developed his own thinking influenced by existentialism. Life was tragic, according to Unamuno, because of the knowledge that we are to die. He explains much of human activity as an attempt to survive, in some form, after our death. Unamuno summarized his personal creed thus: "My religion is to seek for truth in life and for life in truth, even knowing that I shall not find them while I live." He said, "Among men of flesh and bone there have been typical examples of those who possess this tragic sense of life. I recall now Marcus Aurelius, St. Augustine, Pascal, Rousseau, "René", "Obermann", Thomson, Leopardi, Vigny, Lenau, Kleist, Amiel, Quental, Kierkegaard—men burdened with wisdom rather than with knowledge." He provides a stimulating discussion of the differences between faith and reason in his most famous work: "Del sentimiento trágico de la vida" ("The Tragic Sense of Life", 1912).