In a chapter on "The Naturalist and the Supernaturalist" Lewis gives technical definitions to the two terms. Naturalists, under his definition, believe that the Universe is a vast process in which all events which ever happen find their causes solely in the events that happened before them within the system. Supernaturalists believe that interruptions or interferences can take place in this system of our Universe from some other system outside it. In particular, a supernaturalist believes that the natural world was created or derived from a supernatural entity. A supernatural event would be one that is not traceable, even in principle, solely to materially determined causes within our Universe. Libertarian free will, if it exists, would have to be supernatural under this view.