The principal parts of architecture—most evidently the floors, walls and roofs (and less evidently, heat and light)—are seem as a comprehensible system of signs by which we navigate each and every day. A building has to be understandable: on the one hand, it has to be suggestive enough of its purpose for those who would otherwise be disoriented while, on the other hand, it should not be so single-mindedly purposeful as to preclude all its as-yet unimagined uses.