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
Modernist planning was obsessed with absolute numbers, including the minimum dimensions of rooms, open space per capita, and the one-size-fits-all head counts of neighborhood units. This was often pegged at five to seven thousand and was used as a formula for determining the distribution of schools, shops, sports fields, and other facilities. The failure of such planning is not in its effort to be comprehensive or to equalize access to necessary facilities. It is, rather, the attempt to rationalize choice on the basis of a homogeneous set of subjects, a fixed grammar of opportunities, a remorseless segregation of uses, and a scientistic faith in technical analysis and organization that simply excludes diversity, eccentricity, nonconforming beauty, and choice. The utopian nightmare.