Architect: Obviously, a dirty stone building is less attractive than a clean one  But the process of cleaning stone buildings with water drives the water deep into the stone, eventually causing irreparable damage  Dirt also damages stone buildings, but less than water does  So I am afraid stone buildings must remain dirty if they are to last as long as possible  Engineer: Not so fast
Advances in technology make it possible to clean stone buildings without water, using mild chemicals that do not penetrate the stone
showing that the architect's argument is internally inconsistent