Optimism, irrational though it might be, is central to the human psyche: it seems to give us an advantage both in everyday life and in the evolutionary race. What does Ernest Shackleton's Antarctic expedition have in common with the chicken that crossed the road? Or James Baldwin's campaign for civil rights with the development of AI? Or even Crossrail and George Bush's 'mission accomplished'? 'The Bright Side' makes a vital and transformative new argument: that optimism is not only the natural state of humanity, but an essential one.