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Tractatus Logico-Philosophicus (1922)

Tractatus Logico-Philosophicus was written by Wittgenstein in German and published in 1921 as Logisch-Philosophische Abhandlung. (It was translated into English and published with its Latin title in 1922.) Focused on logic and metaphysical topics, it is considered among the most important philosophical works of the 20th century. Its publication also launched a reluctant Wittgenstein as one of the great thinkers of his time. Tractatus Logico-Philosophicus is structured around 525 declarative statements, or propositions, as it examines the roles and the significance of both language and science.

Blue and Brown Books (1958)

The Blue and Brown Books were written by Wittgenstein between 1933 and 1935. The contents of two books are lecture notes that Wittgenstein dictated to his students at Cambridge University, with the Blue Book being dictated in 1933–1934 and the Brown Book in 1934–1935. Both books provide readers with an indication of the direction Wittgenstein’s thinking took during these years, and the Brown Book represents an early draft of ideas that would later appear in Wittgenstein’s posthumously published Philosophical Investigations (1953). The Blue and Brown Books circulated among Wittgenstein’s students and friends for many years before they were bound and published together for the first time in 1958.

On Certainty (1969)

Posthumously published in 1969, On Certainty is a series of notes Wittgenstein took toward the end of his life on matters related to knowledge, doubt, skepticism, and certainty. Although the notes are not organized into any coherent whole, certain themes and preoccupations recur throughout. In On Certainty, Wittgenstein does not try to refute skeptical doubts about the existence of an external world so much as he tries to sidestep them, showing that the doubts themselves do not do the work they are meant to do.

On Certainty is discussed in a single section of Summary & Analysis in the SparkNotes guide Selected Works of Ludwig Wittgenstein.

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