Entering the Portikus exhibition hall, you walked directly toward a solitary dark boulder, split in half down the middle. The boulder is about a yard across, knee-high, and jagged as a nugget of lava. But the split edge is smooth and perfect. Its precision distinctly shows that this cut was carried out deliberately, with carefully calibrated instruments. And while one half looks rusty and a bit weathered, as though it had been outdoors a long time, the other has a metallic sheen; it seems to have been sheltered from the elements.

Though this boulder, which constituted the entirety of the show “