No epoch-making artist simply accepts his or her means of art-making as handed down from previous artists. From Aristophanes to Michelangelo to Shakespeare to Moliere to Picasso to Beckett, all can be seen (sometimes only in retrospect) to have engaged in lifelong critiques of their working media. This is never in itself a reliable indicator of greatness, and in twentieth-century art - which, high and low, good and bad, has been preoccupied with reflexivity - it is an especially poor one. Sometimes, however, an artist's critique is so confident, thoroughgoing and persuasive that it causes significant change in the public's idea of what a particular medium is, or can be. As critics have frequently pointed out, Beckett's stage plays actually changed many people's notions of what can happen, or is supposed to happen, when they enter a theatre.