French composer of classical music. He was himself qualifying himself as a composer of silence and duration.
He is famous for the Gymnopedies, of course, but other incursions into extreme art are notables, for instance, his "vexations", which is a short piece of music, to be played "very slowly" but, crucially, 840 times:
Pour se jouer 840 fois de suite ce motif, il sera bon de se préparer au préalable, et dans le plus grand silence, par des immobilités sérieuses.
Another artist, John Cage, took up the challenge and performed the piece. The Wikipedia reports this outstanding performance as recorded by the Times:
Cage set the admission price at $5 and had a time clock installed in the lobby of the theatre. Each patron checked in with the clock and when leaving the concert, checked out again and received a refund of a nickel for each 20 minutes attended. "In this way," he told Lloyd, "People will understand that the more art you consume, the less it should cost." But Cage had underestimated the length of time the concert would take. It lasted over 18 hours. One person, an actor with The Living Theater, Karl Schenzer, was present for the entire performance.
He is also the founder of the Église Métropolitaine d'Art de Jésus Conducteur.