Bangladesh, July 31 -- uring the seven years I served as chief economic adviser to the government of India and as chief economist at the World Bank, I came to appreciate what a strange discipline economics is. Parts of it amount to little more than organized common sense, which is why political leaders with no formal training can sometimes make policy decisions every bit as sound as those of professional experts.

Yet other parts are as precise as engineering. In these domains, allowing politicians to make policy unaided is like asking an accountant to design a bridge. Some of the gravest policy mistakes occur when political leaders venture into territory that demands professional expertise.

India's disastrous demonetization in 2016, in...