Chesterton, being a very humble man, chose not to talk about himself in portions of this book.  There is much in his account here about the world of his time and his own thoughts.  It is like Augustine's Confessions, being more an autobiography of his ideas than of his actions.

Chesterton starts with his childhood, leads into his slight involvement in occultism, then to his conversion (when he realized that all the things he thought he had discovered by thinking were what Christians had believed all along), and into his literary career and political activities. Along the way he expresses his views on materialism, determinism, naturalism, education, science, Catholicism, evil, art, the common man, ethics, war, politics, truth, writing fiction, optimism and pessimism, nature, human rights, etc. 

He discusses optimism and pessimism near the end of the book. They were a major issue of his time, as Modernism was clashing with despair following the World War and the Great Depression. He concludes that neither is the correct stance.  "The two sins against Hope are presumption and despair," he says, noting that what we should be doing is not presuming that things will go right, nor despairing that they will go ill, but appreciating what we have.

He finished the book right before he died and it was published posthumously.