Wittgenstein's treatise, Philosophical Investigations, represents a collection of many of his philosophical musings produced in manuscript form. It is divided into two parts, the first of which is 693 paragraph-length comments on the nature of language. The second (more experimental) part is divided into longer musings on things such as picture-objects and mathematical principles, as well as his famous "duck rabbit" (a sketch of an animal's head which may be interpreted as a duck or rabbit). The second part was collected after Wittgenstein's death, but the two parts were published together.

In the preface of his Philosophical Investigations, Wittgenstein claims to set out to contradict many of his earlier philosophical contentions laid out in his Tractatus (1921). Specifically, his ideas of language and philosophy have matured in his later life, and it is these developments that he discusses in Philosophical Investigations. First, Wittgenstein discusses the role of philosophy as investigating primarily language. Language ambiguities are the cause of major philosophical debates. Wittgenstein introduces and dismisses St. Augustine's now famous explanation of how he himself learned language (viz. by having elders point to things and repeat the name for them). For Wittgenstein, asking what is a word is akin to asking what is a piece in chess; the piece is only given meaning insofar as it is part of a game.

Wittgenstein claims that meaning lies in the use of a word within language. Therefore, language is more of an activity than a discrete structure to be understood. Wittgenstein's more mature view of language is one of use rather than representation.

Grammar, which is part of these language-games, is a set of rules for what does and does not make sense. According to Wittgenstein, language makes evident the form of life. Put another way, life enables language to function (as there would be no language without living things).

Wittgenstein closes with a general discussion of philosophy itself. Wittgenstein argues that philosophers that seek to arrive at general theses from a priori reasoning are misguided. Instead, philosophy should show us how to resist temptations to engage in this practice.