In the chapter headed The Great Lesson, Geraint is taught to improve his fignting skills, as well as learn archery, but all of this is just a means of learning how to better control his mind. This chapter is loosely based on two Zen stories.
The first is an old parable of the Zen master who was also a master archer. A young upstart had been roaming the country challenging other master archers to competitions, and beating each of them. Eventually he visits the Zen archer and challenges him to a competition.
They match each other, arrow for arrow. When the targets are moved further away, each still hits the target, often piercing the other's arrow. Finally, the Zen archer challenges the younger man to a final test.
Together they climb a high mountain and come to a deep cliff with a fall of many hundreds of feet. The Zen archer coolly takes an arrow from his quiver, and walks to the edge of the cliff, and with his toes over the edge, fires his arrow into the void below. The challenger can barely stand near the cliff edge, let alone approach it and fire his arrow. He is defeated, not by a lack of skill, but by his mind.
The second story is that told by Herrigel in his classic book, Zen in the Art of Archery. Herrigel is a philosophy teacher living in the Japan that existed between the two world wars, and is being taught Archery by a Japanese Master. In many ways he is a very poor student, disbelieving the teachings he gets from his Master, and in particular believing that good archery is only mastering technique.
He is almost thrown out of the course when the Master sees this, and eventually Herrigel comes to realise that there is a level of archery beyond technique, a point where "I" does not exist, where "it" fires the perfect arrow. This is the Great Lesson as taught to Geraint; that somewhere along the path there is a point where one can know the Great Lesson - that all is one. There is no separation between the archer, the arrow, and the target, for they are all part of the One.
This knowing is not a knowing of the head, an understanding of the intellect, rather it is an experience of the divine.