he had found a piece of charcoal under his bunk , and had just finished writing this legend on the wall amid the intaglio of male and female genitals , names , phone numbers , and obscene little poems : i am not the potter , not the potter 's wheel , but the potter 's clay ; is not the value of the shape attained as dependent upon the intrinsic worth of the clay as upon the wheel and the master 's skill ?