Ludwig Wittgenstein drafted the *Tractatus Logico-Philosophicus* as a soldier in the Austro-Hungarian army during the First World War, while serving as a forward observer at the front. He finished it in an Italian prisoner-of-war camp at the end of the war. The German text appeared in a journal in 1921; C. K. Ogden's English translation followed in 1922.

The book is a list of seven main propositions, each elaborated decimally by sub-propositions (1, 1.1, 1.11, 1.12, ...). The architecture is strong, but the meaning is at times cryptic. Wittgenstein's central thesis is this: the world consists not of objects but of facts; sentences are 'logical pictures' of those facts. A sentence is meaningful if and only if it represents a possible state of affairs. Logical syntax draws the boundary of language; to construct a 'sentence' beyond this boundary (ethics, aesthetics, metaphysics) is in fact a grammatical error.

It follows that most of philosophy — the existence of God, good and evil, the meaning of meaning — consists not of questions waiting for answers but of badly-formed expressions. The solution is not to answer such questions but to dissolve them. After this severe position the book ends with: 'Whereof one cannot speak, thereof one must be silent.'

But Wittgenstein leaves room for the 'showable': beyond the limits of language there is a mystical region — that cannot be said but shows itself. This paradox makes the book different from what the positivists (the Vienna Circle) understood; Wittgenstein found their reading mistaken.

After the *Tractatus* Wittgenstein believed he had 'finished' philosophy; for a decade he taught in village schools in Austria. Then he returned to Cambridge and began work on the *Philosophical Investigations*, which would criticise most of the *Tractatus* itself (published posthumously in 1953). Modern analytic philosophy rests on the tension between these two books.