This comprehensive and accessible book is designed for use by students following the Theory of Knowledge course in the International Baccalaureate (IB) Diploma Programme. The book is also useful for students following other critical thinking courses. The fundamental question in Theory of Knowledge is How do you know? In exploring this question, the author encourages critical thinking across a range of subject areas and helps students to ask relevant questions, use language with care and precision, support ideas with evidence, argue coherently and make sound judgements.