The vocabulary of serialism is rooted in set theory, and uses a quasi- (some would say "pseudo-") mathematical language to describe how the basic sets are manipulated to produce the final result. Musical set theory is often used to analyze and compose serial music, but may also be used to study tonal music. According to Boulez, "Classical tonal thought is based on a world defined by gravitation and attraction, serial thought on a world which is perpetually expanding." The latter types of metaphors-- which seek to closely associate contemporary art with contemporary science-- are typical of mid-twentieth century Modern composers.