Hesse's writings are largely confessional and autobiographical, delving into the theme of "Weltanschauung," or a personal philosophy of life. Many of his works, including Siddhartha, center on an individual's search for truth and self-identity through what Hesse describes as the "inward journey." In the story, Siddhartha, a devoted son of a prosperous Brahman, suddenly feels that his life is hollow and that his soul remains unfulfilled despite his dedication to duty and strict religious practices. With his friend Govinda, he leaves home to begin his journey. Initially, Siddhartha becomes an ascetic mendicant, but fasting and physical deprivation do not bring him closer to peace. He then engages in conversation with Gotama Buddha, who has reached Nirvana. However, Siddhartha realizes he cannot embrace Buddhist teachings on liberation from suffering or learn through the Buddha's doctrine. He must carve out his own path. Departing from asceticism, he indulges in a life filled with desire and sensual pleasures, but years later, he once again feels disheartened and empty. In a state of despair, Siddhartha returns to a river he previously crossed. There, he learns from a ferryman to listen to the river, and it is there that he finally finds peace.

In Siddhartha's concluding conversation with Govinda, he tries to express the insights he has gained. These insights include the notion that for every truth, its opposite is equally valid; that excessive searching—as practiced by Govinda—is counterproductive; and that to "find" is, paradoxically, "to be free, to be open, to have no goal." One must simply love and appreciate the world in all its aspects. Although Siddhartha may have reached the highest level of wisdom, he finds himself unable to convey its essence to Govinda. Another realization is that while knowledge can be communicated, wisdom cannot. He tells Govinda, "These are things and one can love things. But one cannot love words…. Perhaps that is what prevents you from finding peace, perhaps there are too many words, for even salvation and virtue. Samsara and Nirvana are only words, Govinda." It is only through an act of love, when Govinda kisses Siddhartha, that he too sees the "continuous stream of faces—hundreds, thousands, which all came and disappeared and yet all seemed to be there at the same time, which all continually changed and renewed themselves and which were yet all Siddhartha."

Although Siddhartha is set in India and incorporates elements of Buddhist philosophy, it would be simplistic to view the book as a portrayal or explanation of Indian philosophy. Written in the aftermath of World War I, Siddhartha reflects Hesse's attempt to restore his faith in humanity, regain his lost peace of mind, and re-establish a harmonious connection with the world. Siddhartha's journey is uniquely his own, distinct from the paths of Govinda, Buddha, or even Hesse himself. Hesse's next major work, Steppenwolf, contrasts sharply with Siddhartha by replacing tranquility with intensity, situating personal issues within a societal context, and highlighting the dichotomy between the "inner" and "outer" worlds for both grotesque and humorous effects.