In the *Tractatus Logico-Philosophicus* of 1921, Wittgenstein had argued that language 'pictures' the world and declared that all problems of philosophy had been solved. He then left philosophy, taught in village schools in Austria, and retreated to a cabin on a Norwegian fjord. He returned to Cambridge in 1929, defended the *Tractatus* as a PhD thesis (G.E. Moore and Bertrand Russell were on the panel) and was accepted. But once back he decided his youthful system had been wrong from end to end.

For the twenty years from the 1930s to his death he built a new philosophy of language — but published no complete book. *Philosophical Investigations* was assembled from his manuscripts after his death by his students G.E.M. Anscombe and Rush Rhees. The book looks nothing like a traditional academic philosophy text: 693 numbered short paragraphs, often phrased as questions, often closer to aphorism.

The central thesis: the meaning of a word is found not in its 'reference to reality' but in its use within the concrete practices of life. Wittgenstein introduces the concept of a 'language game' (Sprachspiel): language is not a single logical structure but the sum of countless small practices (giving orders, asking questions, telling stories, joking, accounting, and so on). Each practice carries its own rules; when a word moves from one game to another its meaning shifts. 'Meaning is use' (Bedeutung ist Gebrauch).

One of the book's most famous sections is the 'private language argument.' Wittgenstein argues that a 'private language' — one referring only to a single person's inner sensations and verifiable by no one else — is logically impossible: without rules there is no language, and rules can only be checked within a community. The consequences for philosophy of mind (Wittgenstein dismantles the Cartesian 'inner theatre' model), epistemology, and philosophy of language are immense.

When the book appeared, analytic philosophy was still largely under the influence of logical positivism; the 'ordinary language philosophy' strand of the *Investigations* was carried at Oxford by J.L. Austin and Gilbert Ryle, and went on to shape philosophy of mind (Searle, Dennett), philosophy of science, even social theory (Peter Winch and anthropology) and theology. *Philosophical Investigations* remains the second most widely taught text in analytic philosophy — only behind Wittgenstein's own *Tractatus*.