What is the number one? How do we know that 2+2=4? These apparently simple questions are in fact notoriously difficult to answer, and in one form or other have occupied philosophers from ancient times to the present. Gottlob Frege's conviction that the truths of arithmetic, and mathematicsmore generally, are derived from self-evident logical truths formed the basis of a systematic project which revolutionized logic, and founded modern analytic philosophy.In this accessible and stimulating introduction, Joan Weiner traces the development of Frege's thought from his invention of a powerful new logical language in Begriffsschrift, through his explication of his project in the Foundations of Arithmetic and famous papers such as 'On Sense andReference', to the brilliant, but ultimately doomed, presentation of the system in Basic Laws of Arithmetic. At each stage, she discusses Frege's motivations in a way which enables the modern reader to appreciate the originality, clarity, and profundity of his thought.