Polarity

Ultimately, Siddhartha learns that the path to enlightenment is intrinsic. However, throughout the story, Siddhartha's journey helps the materialization of his realization.
In the beginning, Siddhartha expresses his restlessness at home as his spiritual curiosity peaks, and he no longer takes what his father presents him at face value. He begins questioning a structured, scheduled lifestyle as the way to enlightenment. He follows the Samanas, and the Vasudeva and tries to learn from them, and on in this way until he reaches satisfaction. He never is told an answer he is quite satisfied with, although at times he is pointed in the right direction, and furthermore, he learns from his mistakes (or rather the mistakes of others). He learns what doesn't work in a sort of process of elimination manner. But in this logic, in a way, he is taking evidence and information from the outside world.
In fact, he is told by his mentor that he must incorporate the surroundings of the world into his pursuit for enlightenment. He discovers the answer in The River. While the answer comes from within, all the relations and parallels and lessons he draws from the river outward manifestations of the truth. Govinda learns in the same way that Siddhartha learns.
This binary between internal and external knowledge works as the path to enlightenment, and it is a balance between the two that help the protagonist achieve success. In similar fashion, we see Siddhartha battle many polarities. In addition to the one mentioned, we specific examples (and applications of the logic-internal vs. external) of him fighting for answers that help in reach his goal, in time and timelessness, love and detachment, etc.