Some jazz musicians resist any attempt at classification. One difficulty is that most jazz has an element of improvisation. Many musicians draw on free jazz concepts and idioms, and free jazz was never entirely distinct from other genres, but free jazz does have some unique characteristics. Pharoah Sanders and John Coltrane used harsh overblowing or other extended techniques to elicit unconventional sounds from their instruments. Like other forms of jazz it places an aesthetic premium on expressing the "voice" or "sound" of the musician, as opposed to the classical tradition in which the performer is seen more as expressing the thoughts of the composer.