This volume covers the period from the beginning of Whitehead and Russell's work on Volume 2 of the Principles of Mathematics to the critical discovery of the theory of descriptions in 1905. The volume contains a large number of unpublished manuscripts which give a vivd picture of Russell wrestling with the logical paradoxes, often unsuccessfully, as he tries out one foundational scheme after another. Previously unpublished work in the theory of denoting is included, which predates the famous article of 1905. This volume also gathers together several manuscripts on the so-called `zig-zag' theory with which Russell attempted to provide a type-free foundation for mathematics. A number of reviews and survey articles, together with two talks on modality and truth, are also published for the first time.