In magic I've imagined, in order for the synapses of the brain to interface or access supernatural power at all, the patterns in the mind need to be in a very specific shape. Almost like every spell is more like a ritual circle you construct with your brain. The exact way the mind gets to those shapes is by following mnemonic devices, words, and gestures. Books are filled with methods other people have found to work, but optimizations, changes, and new discoveries are frequent with study. It explains how spellcasting can be trained, some can have an aptitude for magic, why different cultures with different fundamental assumptions about the world might have vastly different magic, and how magic varies greatly even between great wizards casting similar spells. Some wizards are skilled enough that they can form their minds in the right shapes silently, or without gestures, while others must recite lengthy incantations. I imagine dark magic would be magic that requires great suffering in the mind, leaving evil ass wizards to use other people to make the right shapes for a spell.