Like Chapter 3 "Proof of Skill" has also gone through numerous revisions. Its another exposition chapter and so I've had to change things as I've discovered them. When I started the last major revision I realized just how much was missing and how much I could do to make things clearer for the reader. The mechanics of spiritual power and how they're developed are explained here but more important is Eric's personal development.
From the very beginning I wanted Eric to be a mage because of my realization of how magecraft works. It requires a strong spirit which implies a strong mind and a strong will. Eric had none of them and would have to develop them as he went. Thus, the stronger he became as a mage the more confident he would become and the more confident he became the better mage he would be. Its a direct feedback loop. Of course, there are other reasons to develop Eric's power besides an analogy for confidence building. They're hinted at later in this book but don't become relevant until the second one.