Preface by Bertrand Russell; One of the Greatest mathematician of his age, Poincare here deals with a variety of issues of methodology: the selection of facts for study, the calculation of errors, and the use of statistical methods to compensate for errors, and the use of statistical methods to compensate for errors. It also contains an attack on logicism in the foundations of mathematics, and an early account of the significance of methodology of the "new mechanics" of radioactive decay.