|If you are writing a fantasy short story or novel that includes Magic, have you thought much about its nature, or is it just something that is there to be used whenever you need it? If you have a clear picture in your mind of how Magic works in your fantasy setting, it will help give your writing that consistent feel that readers expect. With this essay, I put forth one possible view of Magic, not necessarily for you to emulate, but with the hope that it will spark some creative flame within you to devise your own concepts of Magic.
I view Magic as akin to Fire. It isn't there until the conditions are right for it. The wizard knows how to bring about those conditions. The right conditions might also happen accidentally, whether it be nature starting a random Fire or creating a random outburst of Magic. The careless handling of Fire can be dangerous, just as can be the careless handling of Magic.
Fire relies on a particular substance, namely oxygen, to be present. This substance is invisible to the naked eye, but is available on a widespread basis. It is not ubiquitous, however, and so you have areas where Fire can not be created. I see Magic as the same. There is a substance, let's call it manna in this essay, that must be present for Magic to exist. There can be areas where manna does not exist, and in those places Magic can not be invoked.
Since manna is (probably) not oxygen, it does not necessarily only exist in the same places as oxygen. But you could say that manna is oxygen if you wanted to define it that way in your world. Then Magic could only be done in the same places where Fire can be created. Or you could say that manna is water. Anywhere that moisture exists, you could invoke Magic. Or you could say that manna comes from the soul, so that anywhere that you find creatures with souls, you could invoke Magic. You could go so far as to say that the planet has a soul, and thus if you are on or near the planet, you have a source of manna, but you could also draw it from yourself or a nearby person if you needed to. If you associate manna with life energy (which may or may not indicate the presence of a soul, depending on your definition of soul), then you could draw manna from animals or plants or yourself (or the planet if you consider the planet to be a living thing).
Just as we scarcely ever worry about depleting the overall supply of oxygen when we start a Fire, so do we scarcely ever worry about depleting the overall supply of manna when we practice Magic. But we can deplete local supplies of either oxygen or manna.
Fire typically consumes the item with which oxygen was combined to create the Fire. But some substances are inflammable, and will not burn when exposed to Fire.
Perhaps Magic also has a consuming effect, and thus burns out items that are not properly prepared. Items that are prepared properly can be exposed to Magic without being consumed by it. Wizards themselves may be of this nature, as are perhaps permanently enchanted items. Less well prepared magic items might turn to dust after first use or a few uses.
Most objects considered inflammable can still be incinerated by a Fire that is hot enough. So it is with Magic. If you direct Magic at a target and that target is not sufficiently protected, then the Magic will effect it.
Oxygen on Earth is required for certain living things to continue living, and is a byproduct of other living things. This creates a symbiotic relationship between the two types of living things. What if it were the same with manna? What if manna is necessary for fey kindred to live, but is a byproduct of mundane kindred? In such a world, what would happen if the mundane kindred evolved so as to no longer produce manna? They might continue on with their lives, while the fey kindred died off.
Do you believe there are fey kindred on our Earth now? If not, might there once have been, but they died off because we mundanes stopped producing manna? If you do think fey kindred are here, whether we mundanes can see them or not, might they be dependent on something that we mundanes produce without knowing? Perhaps the fey need to stay close to mundanes to survive.
Moreover, our very existence might depend on something that the fey produce, and we, with all our science, do not even realize it, because it is something we cannot measure. Perhaps the unseen fey produce emotional energy that boosts our own, and without it we mundanes would be more depressed and angry than many of us already our. It could be that without the fey, we mundanes would devolve into uncivilized packs of primitive mammals.
Once you make certain conjectures about the nature of Magic, you can keep extending them until eventually you have a large set of consistent rules for Magic that will draw your readers into your fantasy world and suspend their disbelief.