Was David Hume radically sceptical about our attempts to understand the world or was he merely approaching philosophical problems from a scientific perspective? Most philosophers today believe that Hume's outlook was more scientific than radically sceptical and that his scepticism was more limited than previously supposed. If these philosophers are correct, then Hume's approach to philosophy mirrors the approach of many contemporary philosophers. This similarity between Hume and many aspects of contemporary philosophy suggests that we should try to understand Hume not as an historical relic but as a partner in a continuing philosophical dialogue. When we look closely at Hume's thoughts about human understanding, we find that Hume's scepticism emerges very insistently in the context of Hume's scientific approach. This book tries to come to terms with Hume's scepticism in a way that sheds light on contemporary philosophy and its relationship to science.