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.

Philosophical Investigations (1953)

Philosophical Investigations was published in 1953, two years after Wittgenstein’s death. The work delivers an extended criticism of the approach to philosophical thinking that focuses on creating metaphysical theories and deep explanations that cut to the core of the concepts that govern human life and reality. Wittgenstein argues that since there, in fact, are no concepts or explanations hiding beneath the surface of everyday phenomena, and because these metaphysical theories are built upon unwarranted assumptions or generalizations (often born out of the structure of our grammar), this approach is deeply flawed. The purpose of Wittgenstein’s philosophy as presented in Philosophical Investigations is to lead us to recognize temptations toward metaphysical thinking and to learn to defeat them.

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.