How far would you go in your search for certainty? And once you get there, how do you convince others? From the medieval Islamic world to the recent pandemic, scientific progress has relied on different methods of establishing fact from fiction. Today, in the face of ever-increasing disinformation, how we prove things - to ourselves and others - has never felt more urgent. But there is far more to proof than axioms, theories and scientific laws: when demonstrating that an experimental medical treatment works, persuading a jury of someone's guilt, or deciding whether to trust a new type of financial transaction, weighing up evidence is rarely simple. Bestselling author, statistician and epidemiologist Adam Kucharski ranges across science, politics, philosophy and economics to explore how truth emerges - and why it falters.