To build a folly is essentially to do something a second time, something at an inopportune moment. That something is always the memory of something forgotten, about which we can paradoxically say "There it is again."
Follies were misunderstood, purposeless constructions. They were often only small, extravagant gestures in a garden, easily whisking off the imagination to distant lands, a sort of time capsule built to awaken the memory and induce surprise in passers-by. They marked locations, organized secondary paths in a park, or simply predicted the arrival of better times—a demarcation, a sacred spot, a mysterious trail, a hill whose tragic rocky nature begged for a tower, a party, or the arrival of summer.
“It is demonstrably true that things cannot be other than as they are. For, everything having been made for a purpose, everything is necessarily for the best purpose.” — Professor Pangloss
There is a story of Rikyu which well illustrates the ideas of cleanliness entertained by the tea masters. Rikyu was watching his son Shoan as he swept and watered the garden path. "Not clean enough," said Rikyu, when Shoan had finished his task, and bade him try again. After a weary hour the son turned to Rikyu: "Father, there is nothing more to be done. The steps have been washed for the third time, the stone lanterns and the trees are well sprinkled with water, moss and lichens are shining with a fresh verdure; not a twig, not a leaf have I left on the ground"
"Young fool," chided the tea master, "that is not the way a garden path should be swept. "Saying this, Rikyu stepped into the garden, shook a tree and scattered over the garden gold and crimson leaves, scraps of the brocade of autumn! What Rikvu demanded was not cleanliness alone, but the beautiful and the natural also.