When Escher's inventiveness stalled, he tackled the obstacle much as a mathematician does an intractable problem -- with obstinacy. Mathematicians pose questions that nag and pester, they keep chipping away at a problem, until truth, a solution, presents itself (or the enterprise crumbles and proves impossible.)

--- S. Roberts, King of Infinite Space, Walker & Company, 2006, cited in http://www.cut-the-knot.org/WhatIs/WhatIsProof.shtml