Magic is a kind of energy. It is given shape by human thoughts and emotions, by imagination. Thoughts define that shape—and words help to define those thoughts. That's why wizards usually use words to help them with their spells. Words provide a sort of insulation as the energy of magic burns through a spell caster's mind.

The Dresden Files Collection 1-6 (ed. Penguin, 2010) - ISBN: 9781101499689