Here we’ll discuss where magic comes from – that is, through what means magic manifests in the world. Generally, magic can be categorized into two sources: inherent and external.
External systems are the far rarer of the two. Indeed, I can only think of two magic systems that fall under this category: Sanderson’s hemalurgy and fabrial systems. In an external system, magic can only be accomplished through means external to the user: that is, through devices and objects.
Inherent systems are far more common. Inherent systems refer to systems of magic where magic originates in the caster. The caster has some kind of mana, or all that is required is some kind of invocation, or so on. Regardless of the equipment involved, as long as the effect flows out of the caster or is primarily controlled by the caster’s mind, a system is inherent.
Sanderson’s feruchemy system, for example, is inherent. While it may seem external, much like its sibling hemalurgy, the two function quite differently in their execution. Both require certain metals to function, but feruchemy is open only to those with the genetic ability to tap into the magic – storage of specific qualities in pieces of metal – and the ability is solely activated by the mental action of the caster. Hemalurgy, meanwhile, occurs regardless of the intention of the caster.
I would go so far as to say all invocational systems are automatically inherent, as invocational systems rely on the language of the caster, and thus magic, as words from the caster’s mouth, flows out from the caster.