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
No kind of shape, no kind of design or kind of picture or other work of art can be beautiful. No kind of color is beautiful. Beauty comes always from the singularity of things. Two things which happen to be closely similar in size, color, insurance value, smell, weight, or shape, may both seem equally beautiful. It is not therefore to be deduced that, say, a smell of turpentine is a necessary prerequisite of beauty; and nor is the fact that the two things' shapes are measurably within a millimeter of each other. They might still be as different as chalk and cheese: they might differ hugely in surface quality so that one lived and the other was dead. One judges a man by what he is, by his individuality, his idiosyncrasy; not by his measurable properties or measurable behavior or by the shape of his nose or the description in his passport. So with a work of art.