When spiritual elements are discussed in conventional literature, they are grouped under the opaque term religion. A more conceptually precise approach is provided by Voegelin's political science and, in particular, by his Shellingian term pneumopathology - a disease of the spirit. While terrorism has been used throughout the ages as a weapon in political struggles, there is a vast difference between the groups who would use these tactics for rational political goals and those seeking more apocalyptic ends. Cooper argues that today's terrorists have a spiritual perversity that causes them to place greater significance on killing than on exploiting political grievances.