Sitting on the deck of the riverboat, he was scribbling aimlessly in his notebook, when he suddenly became aware of the presence of hippopotamuses along the riverbank, and as he looked at these huge and marvelous beasts the thought that all life was sacred came to him. As "the iron door yielded", he wrote in his notebook, "I am will to live in the midst of life that wheels to live." He came to believe that this reverence for life was the central principle of all ethics.