The Blue and Brown Books are transcripts of lecture notes Wittgenstein gave to his students in the early 1930s, shortly after returning to philosophy. They are so named because of the color of the paper they were originally bound in.

The Blue Book criticizes the idea that the meaning of a word resides in some sort of mental act or act of interpretation. Calling meaning a “mental” act is just a means of obscuring the matter. If the meaning of language is a matter of how we use words, we could just as easily say that meaning resides in the voice box as in the head. Rather than identify meaning with a mental act, Wittgenstein identifies meaning with use: the meaning of a word is determined by the way we use it and nothing more.

Wittgenstein attacks the philosophical “craving for generality” that leads philosophers to try to make the most general claims without properly considering particulars. This craving leads philosophers to make such general claims as the Heraclitean doctrine that “all is in flux.” Such claims amount to redefinitions: if all is in flux, for instance, the word stable ceases to have meaning. In their attempts to make grand metaphysical pronouncements, philosophers really just twist language out of shape.

Philosophers also often fail to distinguish between physical impossibility and grammatical impossibility, drawing on false analogies. If A has his mouth closed, it is physically impossible to know whether A has a gold tooth. Analogously, we might say that it is impossible to feel A’s toothache and conclude that the feeling of A’s toothache is a piece of knowledge to which we do not have access. In fact, feeling A’s toothache is a grammatical impossibility: the grammar of the word toothache is such that only the person who has the toothache can feel it. If we think that our knowledge is somehow incomplete because we are unable to feel the pains of others, we are simply showing that we have allowed ourselves to become confused about the grammar of certain words. Toothaches are things that people feel when they have them. Toothaches are not objects of knowledge that we can know or not know.

The Brown Book develops Wittgenstein’s concept of a “language game,” where he develops simpler forms of language to examine more closely the contrasts between different kinds of words. The upshot of these experiments is to show that most attempts to draw general theories of language are misguided and to draw our attention to the diversity of the uses and functions of words. Wittgenstein also examines the many different uses of such words as compare, recognize, and understand, showing that they have a variety of uses, all of which are related, but not in any definite way. He calls the relations between the various uses of a word family resemblances because, like the members of a family, the uses of a word share a certain resemblance but that resemblance is not based in any single feature.

The Brown Book contains a number of other significant ideas that are developed further in the Philosophical Investigations. Wittgenstein discusses rule following, arguing that there is no rock-bottom justification for the rules we follow and that we need not consciously follow or interpret a rule every time we obey a rule. He discusses the word can and the way that misunderstandings regarding this word give us mistaken notions about the past and future. He also discusses the distinction between seeing and seeing as, arguing that we can see a bunch of squiggles on a page as a face, but we cannot see a fork as a fork, since no alternative presents itself. In other words, when philosophers speak of seeing things “as themselves,” in the sense of seeing things in their essence, such statements have no meaning. For example, it would not make sense to speak of seeing someone “as a human being” or “as a person”—there’s no difference between that and how we normally see people.

The Blue and Brown Books represent a strong repudiation of some of the central ideas of the Tractatus Logico-Philosophicus. Wittgenstein’s philosophy from the Blue Book onward is often referred to as his “later philosophy,” in contrast to the “early philosophy” of the Tractatus. While the Tractatus argues that language corresponds to reality by virtue of sharing a common logical form, Wittgenstein’s later philosophy abandons the idea of any abstract link between language and reality. Instead, Wittgenstein asserts that language has meaning simply by virtue of how it is used. Unlike the Tractatus, Wittgenstein’s later philosophy does not present a grand, tidy theory that explains how everything falls into place. Instead, the later philosophy is profoundly antitheoretical and unapologetically asserts that there is no way to tidy up the various aspects of language and experience into a single, unified whole. However, the similarities between Wittgenstein’s early work and his later work are possibly more revealing than the differences. Throughout his work, Wittgenstein asserts his conviction that the problems of philosophy only arise through confusion and that a proper understanding of the matter at hand will not answer philosophical problems so much as it will make the problems vanish.

The Brown Book marks the peak of Wittgenstein’s interest in language games and their usefulness as a tool for attacking the idea of fixed meaning. Wittgenstein is wary of theories of language, fearing that they are too simplistic. Any attempt at discussing how words have meaning is liable to assert that there is a single, fundamental link between language and reality and that through this link the meanings of words are fixed in place. One of Wittgenstein’s fundamental ideas is that words do not have fixed meanings but rather carry a family of related meanings. Wittgenstein develops the concept of language games as a tool for counteracting the tendency toward theorizing about language. While theories of language seek to find unity in diversity, language games are diverse by their very nature. Wittgenstein invents series upon series of simpler forms of language, not to highlight the commonalities between all of them but to reveal the irresolvable differences between them. Language games are his tool for showing that no single theory of language can possibly account for the diversity of linguistic phenomena.