I’ve talked before about sci-fi, and how the technology can be divided into hard and soft. The same can be done with magic. And like hard and soft sci-fi, both have their advantages and disadvantages. So, what divides the two?
Hard magic represents magic as technology, albeit very advanced. Hard magic has strict rules, and behaves in predictable ways. If one knows enough about hard magic, they can shape it to their will, just like technology. A good example of this is the magic system from the anime/manga Full Metal Alchemist. In it, magic can be used to reshape any material, but only with sufficient material. If one wanted to make a tree into a boat, the tree would need enough wood. Hard magic systems do have the disadvantage of typically requiring more work to create.
Soft magic is magic in its most arcane form. Magic cannot be predicted, and it is not internally consistent. No one understands its true nature. Soft magic systems do not have to follow any specific rule. A good example of this would be the magic from the world of Tolkein. The boundaries of magic are never defined, and it manifests in many different ways: giants that defy physics, swords that glow in the presence of goblins, etc. The magic cannot be constrained, or consistently catalogued. Another element that adds to this is the presence of gods. If a world has one or more gods who can change reality at a whim, a hard magic system would be difficult to reasonably implement.
One major disadvantage of soft magic is the presence of a deus ex machina. See my previous post for more on this. A magic system that isn’t very well defined can run the risk of making solutions to problems feel contrived. Let’s return to our fantasy hero, Sebastian. He is cornered by the Dark Lord and has no more options. But wait! He looks within, and summons the POWER OF LOVE to invoke some new spell never seen before to obliterate the Dark Lord. Of course, this doesn’t make soft magic all bad. As long as the author writes carefully, tension can be retained without too much trouble.
The things I have described are on two ends of a spectrum. The magic in the Harry Potter series, for example, is a mix of the two. Magic needs an action or verbal command to perform, but the spells themselves are numerous and not very consistent.
Again, magic need not be limited by these constraints. If you disagree with my definitions, feel free to do so. Magic is, after all, vague by definition.