Every cathedral, like any great stone building, is a work in progress. No sooner have the walls risen than they start to collapse, the weight of stone pushing down and splaying out, settling and cracking.
Take a closer look at most great old churches, and you see huge pillars wrapped in metal, iron reinforcing bars embedded in the walls, arches pulled together at their base with metal rods. If you took an X-ray of the buildings, they would look a bit like the mouth of someone who has had a lot of dental work — a messy confusion of interventions, repairs and misguided improvements.