Spiritual Quest
The theme of spiritual quest permeates the novel. All the major characters, with the exception of Kamaswami, have spiritual desires and seek enlightenment. This applies even to the courtesan, Kamala. Various routes to spiritual fulfillment are explored. Siddhartha's Brahmin father relies on the traditional sacrificial rites prescribed in the Vedas. Siddhartha rejects these rites, and he and Govinda experiment by leading the lives of wandering ascetics. Govinda becomes a follower of the Buddha, as does Kamala. Siddhartha is the most determined seeker of them all, and he is determined to pursue his quest in his own way, based on his own experience, rather than accept guidance from a teacher. Siddhartha gets sidetracked for many years when his involvement with the courtesan Kamala and his accumulation of riches as a merchant dulls his spiritual sense. But in middle age he rediscovers the fervent desire for enlightenment that he had known in his youth. He is fortunate in meeting a humble ferryman named Vasudeva who quietly lives the reality of enlightenment rather than preaching about it. He learns much from Vasudeva. In the end, all those who seek enlightenment find it. It is significant that Govinda and Kamala become enlightened through Siddhartha's help; it is he who communicates to them the profound peace and all-embracing knowledge that they seek.

Siddhartha as the Buddha
In the novel, Siddhartha and Gotama the Buddha are two separate characters. But the historical Buddha was also known as Siddhartha, and the character Siddhartha in the novel is also meant to be an exploration of the life of the Buddha, as imagined by Hesse. There are many similarities between Siddhartha and the historical Buddha. Both catch sight of Samanas (ascetics) when they are young and decide to leave their families. They both become wandering ascetics. Both are dissatisfied with the teachers they encounter; both give up their ascetic practices, both become enlightened by the river. In the end, as Govinda discovers, Siddhartha and the Buddha are indistinguishable from each other.

Enlightenment
The basic premise of the book is that life is not merely material. It is not merely what we perceive with our senses or think with our minds or experience with our emotions. There is another dimension of life that is ordinarily hidden from view, but which can be experienced in a state of meditation. During meditation, the mind and senses withdraw from the outer world and perceive the innermost truth. This inner truth is described as Brahman. It is infinite, silent, and boundless. It is also blissful. Brahman is said to be identical with the innermost essence of every human. When the individual experiences Brahman, he knows that the material world is not the reality; it is maya, the play of illusion. This knowledge makes a person free; he or she no longer identifies with all the joys, sorrows, pleasures, griefs and fears of the small individual ego. Unaffected by such transient things, the enlightened person knows that the true reality of life is indestructible and eternal.

This is the doctrine elaborated in the Indian scriptures known as the Upanishads, with which Siddhartha is intimately acquainted. Although the ideas may seem remote from most people's normal experience, mystics throughout the ages, and in every religious tradition, have spoken of experiencing a similar, if not identical, state of being. They emphasize, as Hesse does through Siddhartha in the novel, that enlightenment is a state that must be experienced directly. It cannot be described in words.