Hesse is profoundly fascinated by the duality of existence, especially the domains of the mind, which he refers to as "Geist," and the physical world, which he names "Natur." Siddhartha delves into and exhausts both of these realms. In his father's home, he develops his intellect. With the Samanas, he pursues truth through meditation and severe bodily deprivation. When these pursuits fail to bring him peace, he shifts to the other extreme. He immerses himself in material and sensual pleasures, but this life focused on the body also leaves him unfulfilled.

By the river, Siddhartha discovers a way to transcend both the mind and body, uncovering a third path—the journey of the soul. This synthesis distinguishes Hesse's Siddhartha from Buddha. For Hesse, the river engages with both realms; it is not just an obstacle to be overcome, as in Buddhist symbolism. Instead, Siddhartha becomes a ferryman who connects both sides of the river, representing the natural union of extremes.