So I'm currently trying to write a fantasy story that straddles the spirit of renaissance science, but requires some subtle form of magic. So far the best way to work it in is to make up something called Salt and work it in like that, but I'm wondering if people could make suggestions to improve the concept. The idea is that in the setting of my story, the world is made up of city states that all exercise sovereign rule, and their own borders. Everyone has their own laws and borders, but they need a common currency. Salt acts as that currency. It's not like common table salt, but the apparent manifestation of the energy and effort of people's labours. I haven't quite worked out how it's produced, but vague ideas of vast arenas of slaves being forced to do backbreaking and pointless work to generate it is on my mind. Anyway, the general belief in the world is that Salt is this manifestation of energy and power. Certain people can turn it back into energy and use that to do what they like, whether it's growing a tree in a second or creating a burning fireball, they do it by taking salt and converting it to useable energy. The thing is that since Salt is also the standard of currency, it's pretty much like burning money to do what you want. It's not generally considered a practical solution, so magic exists, but only the rich have access to it. Another drawback of Salt would be that overuse of it leads to massive health problems, and the most powerful Salters (I guess that's the equivalent of a mage in this universe) run the risk of death from overuse if they use too much too quickly. That's what the people believe. My MC however is a reality cleaving bastard who recognises that the Salters are unwittingly exploiting the mass-energy equivalency (it's real physics!). Salt has certain properties that lets them do this without having to know how, but its massively inefficient. Most of it ends up in their bloodstream without being converted, and that's why Salters end up being plagued with ill health (high blood pressure, kidney failure etc.) My MC knows this, and he also knows that just about any material can be made into energy for magic if you understand it, which puts him in the unique position of being able to do much greater magic with anything. Anyway, that's the concept, what do you all think?