Jane Jacobs, who has died at the age of 89, was a giant among analysts of the fate and role of cities. She spent much of her career fighting for one deceptively simple principle: leave cities alone and let them develop by themselves.

Yet her contribution to the world of ideas went far wider than this. A genius – no other word will do – she had the defining quality of any brilliant intellect: the ability to look at the world in a fresh way. She had the knack of asking illuminating questions and coming up with perceptive, original, compelling, and above all correct, answers. Despite her lack of formal training she educated even Nobel prize-winning economists.