Trusting the keystone

An arch built with stones is built with scaffolding.

The final stone, the one on top, is called the keystone. Only when the keystone is placed does the arch stand on its own, and the scaffolding can be removed.

Spectators criticising the scaffolding and the incompleteness miss the point: The builder knows that the keystone will complete and stabilise a new place from which to build further.