Excerpts are taken from Hertzflet's Book on Theory Of Magic, a Beginners Practical

...My students would ask me "How can everyone use magic, but not everyone can use the same spells?"
And frankly, it's hard to answer that question. Anyone can technically visualize and pray really hard and get magic to take effect, but not everyone has that extra...calling to draw the power forth like a muscle...

 ...While there are no limitations to what magic can achieve being only limited to the imagination, will, and beliefs of the user, we teach ourselves limitations all the time. Because our imaginations are not infinite, we all close our mind to some possibilities, and will is a funny conversation in and of itself. SO it's difficult to say what is literally possible in magic, and what is not. Because how can you know what you can do, if all you've done is what you can think of doing?...

...It takes a force of will, and strength of spirit to use magic. You can wish all you want, but if you have a weak mind or body, you'll collapse quickly when using greater magics. It's easy to toss out a simple cantrip or jinx the housewife at the well. But if you want to close a wound, you have to know what is wrong with the wound. You have to know how the wound should heal. What goes where and how. Otherwise, you'll exhaust yourself...and just make things worse.

...People usually specialize in one form of magic or another. As my readers should know, I work almost exclusively with theoretical magic. Learning what is possible, how you can push the limits of what can be done. The Great Powers know how I struggle at times trying to apply this knowledge. There was this time where I...but I digress. When you start specializing, sure you start limiting what you can do. But you learn a single specific level that most mages just can't reach. So you give up other skills in exchange for being the best. My ability to divine the truth is unparalleled in this Kingdom, and most others if I can boast here...