This witty yet thoughtful book examines stupidity and idiocy, and their most dangerous incarnation: stupid idiocy – the unthinking embrace of flawed thinking. While philosophers chase wisdom, this book takes a humbler route, asking instead how we might think just a little less badly. Drawing sharp distinctions between not thinking and thinking poorly, and doubling down on bad thought, it reveals how deeply these failings are tied to our mental habits. With clarity and humour, it considers how stupidity shapes everyday life and public discourse. Within its pages is a chapter on politics and a pointed critique of Donald Trump, ‘stupid idiot supreme’. This is an accessible philosophical guide for anyone who wants to better understand – and perhaps reduce – their own foolishness.