Common sense underpins the constellation of decisions and opinions we make and form every day. As individuals, and as a society, the way we behave, react and interact is grounded in our perception of 'the sensible thing to do'. But history is a litany of the insensible; misunderstandings and failures, wastes of money and in some cases, wastes of life. And yet we continue to trust in the infallibility of common sense. In this bold thesis, renowned network scientist Duncan J. Watts exposes the complex mechanics of judgement and proposes a radical new way of thinking about human behaviour.