Michael Craig-Martin has had a paradoxical career. He is, I think, a disciple of Marcel Duchamp. But the latter famously gave up painting in favour of something more conceptual — ready-mades and whatnot — whereas Craig-Martin began with Duchampian concepts. He once exhibited a glass of water on a shelf together with a claim that he had mentally transformed these, by a kind of transubstantiation, into an oak tree.