Escher's is an art of pure reason: brilliant, cerebral, and filled with humor , though devoid of warmth or spontaneity, as shown dramatically in this collection. The artist's vision was one of controlled, mathematical ecstasy. Best known for his uniquely radical examination of time, space, and perspective, he sought to define reality anew, and - through his endless staircases and other impossible constructions - to transcend the limits of physicality. ''(T)here is no proof,'' he wrote, ''. . . of an objective reality apart from our senses, and I do not see why we should accept the outside world as such solely by virtue of our senses.''