This volume of essays tackles the main problem that arises when considering an epistemology for mathematics, the nature and sources of mathematical justification. Focusing both on particular and general issues, the essays from leading philosophers of mathematics raise important issues for our current understanding of mathematics. Is mathematical justification a priori or a posteriori ? What role, if any, does logic play in mathematical reasoning or inference? And how epistemologically important is the formalizability of proof? Michael Detlefsen has brought together an outstanding collection of essays, only two of which have previously appeared. It will be essential for philosophers and historians of mathematics interested in the nature of reasoning and justification. The companion volume Proof, Knowledge and Formalization is also available from Routledge.