In 1921, the Austrian-born philosopher Ludwig Wittgenstein published a short, dense work called the Tractatus Logico-Philosophicus. It was about 75 pages of numbered propositions, each more compressed than ordinary prose. Wittgenstein believed the book had solved every meaningful problem of philosophy.

The Tractatus argued that meaningful language has a fixed logical structure. Sentences are pictures of possible facts in the world, and anything that cannot be expressed in this picture-form — ethics, aesthetics, the meaning of life, philosophy itself — cannot be meaningfully spoken about. The book famously ends with the line: "Whereof one cannot speak, thereof one must be silent."

Convinced philosophy was now finished, Wittgenstein left academic life. He spent six years as a primary school teacher in rural Austrian villages. He helped design a modernist house for his sister in Vienna, working alongside the architect Paul Engelmann. He spent time as a gardener at a monastery near the city.

In 1929, he returned to Cambridge. Within a few years, he was publicly arguing that the Tractatus had been deeply mistaken.

His later view, developed in lectures and unpublished manuscripts through the 1930s and 1940s, was that meaning is not a fixed structure that mirrors reality. Words mean what they do because of how they are used in the shared practices of human life. He called these practices language-games. Concepts like "knowing," "pain," or "game" itself, he argued, do not have a single defining essence. They have only what he called family resemblances among uses.

Wittgenstein died in 1951. His later work was published posthumously in 1953 as Philosophical Investigations.

Both books are now considered foundational works of twentieth-century philosophy. Both are by the same person. They disagree.