This book examines the concept of post-truth and the impact it is having on contemporary life, bringing out both its philosophical and political dimensions. Post-truth is contextualised within the philosophical discourse of truth, with particular reference to theories of scepticism and relativism, to explore whether it can take advantage of these to claim any intellectual credibility. Sim argues that post-truth cannot be defended on either sceptical or relativistic grounds – even those provided by recent iconoclastic philosophical movements such as poststructuralism and postmodernism. The affinity between post-truth and conspiracy theory is emphasised, and the extent to which post-truth plays a role in religious doctrine is also considered. Post-truth is seen to constitute a threat to liberal democratic ideals and our Enlightenment heritage, raising the question of whether we are moving into a post-liberal age where the far right would hold power. To prevent this, post-truth urgently needs to be countered.