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
If all evidence of civilization on Earth was destroyed, and humans had to re-build society from the ground up, what would be different? Feynman reckons that pivotal scientific moments, like the discovery of the atom, will still happen in the same way. Perhaps mathematics will be similarly rediscovered.
Someone told me once in response to this question, no artwork would ever be recreated. The art we create – music, stories, dance, film – isn’t a fundamental element of the universe, or even of humanity. It’s unique to each artist. If you choose to create art, you leave something in the world that has never had a chance to exist before, and will never again have a chance to exist. There will never be another Beatles or Studio Ghibli or Picasso. Art, in its infinite variations of originality, is cosmically unique in a way the sciences will never be. Art immortalizes human experiences that would otherwise vanish in time.