For a work of conceptual art, Roelof Louw’s Soul City (Pyramid of Oranges) (1967) is unusually generous. Some 5,800 oranges are stacked neatly in a wooden frame on the floor of a gallery. Then visitors are allowed to help themselves to the fruit. By the time I saw it in the big conceptual art show at Tate Britain in 2016, the pyramid had dwindled: it looked more like a ball pit for toddlers, as improvised by a greengrocer. Louw’s oranges, in their abundance and with their more or less uniform shapes, are everyday things; you might see them piled like this, if not in such quantities, on any high street in Britain. But perhaps, in carrying them into the unfamiliar setting of a gallery and then constructing a cairn out of them,