Tractatus Logico-Philosophicus
In August 1918 Wittgenstein completed his "Tractatus Logico-Philosophicus", first published in 1921 in Germany, and widely regarded as one of the most important works of 20th-century philosophy. After military service during the First World War, he decided to become an elementary school teacher. One of his sisters said this was like using a precision instrument to open crates.