We live in an age of mistrust: in governments, companies and institutions of all sorts. Mistrust can be corrosive, encouraging us to turn away from politics or from public life. But mistrust can be a powerful positive force, motivating us to take up the task of keeping watch on those in power, or building new institutions that are more just than those we mistrust. Mistrust may be our most valuable civic asset, if we can learn to harness it to change the world for the better.