In music, especially folk and popular music, a matrix is an element of variations which does not change. The term was derived from use in musical writings and from Arthur Koestler's The Act of Creation, who defines creativity as the bisociation of two sets of ideas or matrices. Musical matrices may be combined in any number, usually more than two, and may be — and must be for analysis — broken down into smaller ones. They may be intended by the composer and perceived by the listener, or they may not, and they may be purposefully ambiguous.