Ticket for the Munich metro – for a transport system that doesn’t require anyone to show their tickets during normal use. The cost of managing do-you-have-the-right-fare infrastructure, versus the level of free-riders. And the other benefits to society in having a system that supports a higher degree of flexibility.

Take anything you currently pay for: food, drink, drugs, rock-n-roll: in which contexts does an honour-system make sense? And in which would it be abused?