A law professor and, until recently, a regulatory regimes adviser to Barack Obama, Sunstein co-authored the bestseller , which helped popularise such ideas as libertarian paternalism and "choice architecture". People like Conly and Sunstein think Mill got it all wrong about individual autonomy and political freedom.

People should be free to do what they like, Mill argues in , because they know what's best for them better than any outsider, say, a nanny state. He then goes to extremes, arguing people may harm themselves, as long as they don't harm others. This is an idea that underpins democratic practices, free markets and deregulation in the English-speaking world, though less so under the European welfare state.

But what behavioural psychology and economics find is that people often don't know what's best for them and suffer the same mistakes and blind spots - even after these have been pointed out to them again and again.

The "present bias", for example, explains why many don't save or save enough for future retirement. So for Sunstein, a responsible government should devise ways to "nudge" people to save for retirement. For Conly, it should make pension-saving mandatory. Likewise, he would encourage more attractive displays of fruit and vegetables in school canteens; she would ban junk food.

Sunstein agonises over whether governments have the right to impose what's best for citizens. Confucius should ease his mind. Like parents lording it over kids for their own benefit, rulers, the sage would say, not only have the right but the obligation to do so.