Disclaimer: This is just a theory and Some if not all of it could be wrong incorrect. The purpose of this post is to get feedback on its components. Ive come up with this myself but it is possible (maybe likely?) that someone has come up with this before me.
The Idea: Magic can only influence the probability of events that could occur naturally.
Broader Idea: If something could not happen naturally then magic can not make it happen. If something will happen no matter what naturally then magic can not stop it.
Example: Death can not be avoided naturally. All things die, Thus magic can not itself prevent death. This does not mean magic cant be used to enhance health and longevity as these are perfectly possible through natural methods.
Definitions: The definitions of key words in this theory are necessary to fully understand the nature of the theory
Magic- Any combination of will, focus, and intent which causes a change in how the world interacts.
Naturally- A hard term to define as there is a very blurry line between natural and unnatural, but in general anything that could happen through nonartificial means.
Influence- Change, Skew
Expanded Idea: If magic can not make something not happen that will happen and can not make something that wont happen occur then magic itself can only influence not guarantee effects.
Example: Using magic to find a job can not guarantee a job. It can influence (increase or decrease) your chance to get a job. If you have not tried to get a job and you would never get a job naturally then magic will not help. If inversely you are getting a job no matter what then magic would be of little use in losing the job. This also seems to say that even if you have a 99% chance of finding a job magic can not make that a 100%, but only increase the chance.
Possible Rule Breakers: In this section ill list some things that can possible hurt the above theory
entities such as spirits, demons, angels, deities ect. and interactions between them- These seem to break the core of the theory as the theory itself has no way of explaining this entire field of magic.
Clairvoyance, Visions, and other psychic phenomenon- These seem to break the theory as they seem to not be something that could occur through natural means. Also they seem as either a they occur or don't occur and there seems to be no in between.
Possible explanations- Perhaps there is chance involved but because of the nature of the abilities this is hard to see. This could also be a natural phenomenon that seems rarer then it really is (example Deja Vu).
So there it is my basic theory. I would love to hear your thoughts on it both positive and negative. It for me at least seems to answer many questions for me involving magic but ill admit there is a lot it cant explain.