Tractatus Logico-Philosophicus is the only book-length philosophical work published by the Austrian philosopher Ludwig Wittgenstein during his lifetime. He wrote it as a soldier and a prisoner of war during World War I. The slim volume (fewer than eighty pages) comprises a system of short statements, numbered 1, 1.1, 1.11, 1.12, etc., through to 7, intended to be such that 1.1 is a comment on or elaboration of 1, 1.11 and 1.12 comments on 1.1, and so forth. It is an ambitious project to identify the relationship between language and reality and to define the limits of science. (Summary from Wikipedia)