Having had to encounter single-handed during his period of eclipse many physical dangers, he was well aware of the most dangerous element common to them all: of the crushing, paralysing sense of human littleness, which is what really defeats a human struggling with natural forces, alone, far from the eyes of his fellows.
Jim is a man living by a code of honor. His life goal is to put himself at risk in order to save another human being. Many critics believe Jim is a Messiah-like figure in this complex narrative that addresses the human struggle to do right or live honorably in a dishonorable society that does not value self-sacrifice.
Jim's main trouble is that in a very real sense he has fallen from grace, yet he is the one man struggling to live a moral live in spite of his failings. Moving away from a perceived condemning society, Jim finds himself on an island, hailed as a lord.
The general idea of the book is that the meaning of life, Jim's life in particular, is beyond human explanation.