The Rinzai Zen temple Ryoanji -- Temple of the Peaceful Dragon -- was built in Kyoto in the late 1400s, and the garden may be nearly as old. Its fifteen moss-covered boulders are placed so that, viewed from any point, only fourteen of the boulders are visible. Tradition says only the enlightened see all fifteen boulders.
The garden invites us to contemplate imperfection and limitation. We may know there are fifteen stones, yet in the imperfect world we do not see them all. In this way, the garden becomes a visual koan.