This collection contains hitherto unknown letters exchanged between Wittgenstein and the most important of his Cambridge friends and includes editorial notes based on archival material not previously explored. Wittgenstein's correspondents are not his disciples, but those he recognised and valued as his mentors - Russell, Moore, Keynes, Ramsey and (represented by a single but important letter) Piero Sraffa.The collection provides a fascinating glimpse of the philosopher - Wittgenstein appears in turn withdrawn and affectionate, fierce and censorious, happy to collaborate and sure of his own judgement. Four quarrels and four reconciliations are documented, along with Wittgenstein's struggles to publish his Tractatus, his retreat from the world and his eventual return to philosophy. It will prove a remarkable read for both students and academics interested in Wittgenstein's work and his place in twentieth-century philosophy.