Bertrand Russell is the most important philosopher of mathematics of the twentieth century. The author of The Principles of Mathematics, and, with Alfred Whitehead, the massive Principia Mathematica, Russell brought together his formidable knowledge of the subject and his skills as a gifted communicator to provide a classic introduction to the philosophy of mathematics. Introduction to Mathematical Philosophy sets out in a lucid and non-technical way the main ideas of Principia Mathematica. It is as inspiring and useful to the beginner as it was when it was first published in 1919.