The broadest definition of music is concerned with sound and silence, and their progression through time. These two elements may be organized in an almost infinite variety of ways ranging from complete randomness to highly constructed systems. Music is foremost a form of human expression which may be made as simple or complex as an individual or group chooses. The diversity of music across cultures and history is extremely broad.
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 presupposition 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 death metal and grindcore, which enjoy an extensive underground following, indicate that even the harshest sounds can be considered music if the listener is so inclined.
20th century composer John Cage disagreed with the notion that music must consist of pleasant, discernible melodies. Instead, he argued that any sounds we can hear can be music, saying, for example, "There is no noise, only sound,". According to musicologist Jean-Jacques Nattiez, "the border between music and noise is always culturally defined--which implies that, even within a single society, this border does not always pass through the same place; in short, there is rarely a consensus.... By all accounts there is no single and intercultural universal concept defining what music might be."
