he knows from experience how to age blood - how , upon leaving the body and splashing on a brick wall , for example , blood will go from red to brown to black to grey , how it will flake off , eventually , in the desert heat , or how it will rehydrate and run in the rain , how it will eventually disappear altogether , leaving only a stain like the dirt of the earth - and how , long after that , even the stain itself will evaporate , because the elixirs of human life are unstable - because human life itself wants to merge again with the elemental world it was born out of and is kept separate only by the puny will of individual fancies .