Magic
In order to understand what is forbidden and what is permitted, first you must understand the fundamental nature of magic, and the divide that stands at its center. All conscious organisms, that is, all organisms in possession of a soul, are capable of influencing the physical world with that soul. In fact, that is the method by which one's soul interacts with one's body – a non-physical entity interacting with a physical one. Ever since the Midnight Age, the soul's inherent magic has been able to easily influence the physical world outside of the nervous system, much in the same way that the actions of the body are made. You do not mentally imagine the mechanisms of electricity and muscle contraction that cause you to grab a salt shaker – instead, you will that action into being, through instinctive knowledge, and much in the same way, you can do the same with direct manipulation of the physical world via the soul. Consider, however, how those muscles and nerves function. By using them, they grow stronger, to support their utilization. The same is true of magical exertion. In closing the gap between what one wills and what is, the soul interfaces with the world, and alters itself to better accomplish that task. That alteration is fundamental to our world. Humans and animals who give themselves over to that alteration create the aberrations we know as demons and monsters, respectively. In order to prevent that, we draw a line between two kinds of magic: Low Magic, or magic in which the task accomplished exists within the physical capabilities of the user, and, High Magic, or magic in which the task accomplished exceeds the physical capabilities of the user. Low Magic is safe, and permitted. High Magic is not, and thus, forbidden.