A concise collection of correspondence that illuminates and defends the core ideas of the Tractatus, clarifying the picture theory of language, the boundary between what can be said and what must be shown, and the limits of propositional discourse; the letters respond to critics, untangle technical points of logic and notation, and explore the ethical and mystical implications that the author regards as lying beyond ordinary philosophical statement, offering a more personal view of his aims and the methodological lessons he drew for philosophy.