Music is a form of art that involves organized and audible sounds and silence. It is normally expressed in terms of pitch, rhythm, and the quality of sound. Music may also involve complex generative forms in time through the construction of patterns and combinations of natural stimuli, principally sound. Music may be used for artistic or aesthetic, communicative, entertainment, or ceremonial purposes. The definition of what constitutes music varies according to culture and social context. Music is a personal response to vibration since the same piece of music will affect people differently. Although it cannot contain emotions, it is sometimes designed to manipulate and transform the emotion of the listener/listeners. Music created for movies is a good example of its use to manipulate emotions. Greek philosophers and medieval theorists defined music as tones ordered horizontally as melodies, and vertically as harmonies. Music theory, within this realm, is studied with the pre-supposition that music is orderly and often pleasant to hear. However, in the 20th century, composers challenged the notion that music had to be pleasant by creating music that explored harsher, darker timbres. The existence of some modern-day genres such as grindcore and noise music, which enjoy an extensive underground following, indicate that even the crudest noises can be considered music if the listener is so inclined.