In zen teaching a koan is an eternal truth expressed through a conundrum, such what is the sound of one hand clapping? when the zen student comes to his master for instruction, he is given first a koan to fathom and to sweep away the clouds of unknowing, allowing enlightenment to occur. Recorded and handed down from master to master, monastery to monastery, the koans in this selection are organized in order of ascending difficulty, and accompanied by brief explanations. The book is one of a three-volume series which also includes " the poetry of zen" and " the wisdom of zen".