What to do with the empty plinth in Trafalgar Square? It was a question that bothered officialdom for a century and a half. Statues on plinths have a way of focusing discontent, as Tate Britain’s exhibition “Art under Attack” recently reminded us, and no public figure seemed sufficiently worthy – or uncontentious – to commemorate in such a key location. Then to make matters worse, in the 1960s statues went out of fashion. With the abstract ­revolution initiated by Anthony Caro, modern sculpture came down off its plinth and its high horse. The “Fourth Plinth” appeared doomed to remain empty, until in 1998 the Royal Society of Arts had the novel idea of using it as a temporary showcase for contemporary commissions.Of the nine sculptures to occup