"For a large class of cases—though not for all—in which we employ the word 'meaning' it can be defined thus: the meaning of a word is its use in the language." (Part I, section 43)
Wittgenstein challenges the notion of fixity of meaning. He reacts strongly against the idea that all (or even most) words have a single fixed meaning that determines how they are used. According to Wittgenstein, use comes before meaning. In particular, he rebels against the idea that "meaning" is a mental state or process that accompanies the words we say. We do not need some inner criterion to fix the meanings of our words: so long as we all use words in the same way and can grasp how other people use words, then communication is unproblematic. Wittgenstein is hesitant to say that the meaning of a word is always its use in language because his attack on fixity of meaning carries with it the recognition that words can be put to a variety of different uses in different circumstances.
"We must do away with all explanation, and description alone must take its place." (Part I, section 109)
Traditional philosophy generally attempts to explain the phenomena it studies, be it language, the mind, or metaphysics. Wittgenstein identifies the shortcomings of traditional philosophy in this drive toward explanation. In looking for explanations, we assume our ordinary language is somehow at fault, and try to dig beneath it to find its inner workings. Wittgenstein suggests that our true error comes from not properly understanding the surface features of our language, and mistaking them to be pointing toward deeper phenomena that must be uncovered and explained. Instead of explanation, Wittgenstein proposes a philosophy of pure description: we must do no more than investigate and describe the varieties of ways in which we speak. This descriptive method will bring to light the variety of different ways we use words like "meaning," thus disabusing us of the notion that there must be a single concept of "meaning" that must be discovered and explained.
"If language is to be a means of communication there must be agreement not only in definitions but also (queer as this may sound) in judgments." (Part I, section 242)
How would we communicate with someone who genuinely doubted that the world existed five minutes ago? We could not discuss history, memory, or current affairs. our very conception of the significance of our lives would be incomprehensible to this person. Further, there is no evidence we could convince this person otherwise. The ordinary statements of everyday life—the kinds of statements that we can claim to know, doubt, justify, and explain—are not founded upon a bedrock of certainty. Rather, they are founded upon a number of judgments that are not open to question. We are able to debate questions in history because we are not able to debate the question of whether the world has existed for more than five minutes. In order to communicate fruitfully, we must agree in these judgments. This agreement is not explicit, nor even tacit. Such agreement reflects our shared forms of life.
"My aim is: to teach you to pass from a piece of disguised nonsense to something that is patent nonsense." (Part I, section 464)
Traditional philosophy, according to Wittgenstein, is made up of "disguised nonsense." He believes that philosophers debate theories that seem plausible when set forward in a philosophical context. However, the very formulation of these theories is founded on unwarranted assumptions and generalizations that we fail to notice because they take place at such a fundamental level. Philosophical Investigations does not attack these sorts of theories head on, but rather probes the fundamental assumptions and generalizations that motivate them. At such a fundamental stage, the error is far more evident, and so far easier to expose. At this level we are dealing with "patent" nonsense. The purpose of the Investigations is to show that this patent nonsense is the very stuff that makes philosophical theories seem plausible.
"If God had looked into our minds he would not have been able to see there whom we were speaking of." (Part II, p. 217)
A great deal of the Investigations are devoted to the philosophy of mind. Wittgenstein wants to show us that words like "meaning," "understanding," "believing" and so on do not refer to any distinct mental state or process. Further, it is inconceivable that they should, as none of the criteria we use to justify talk about mental phenomena appeal to inner states or processes. Psychologists have a mistaken conception of the mind when they think it is only a matter of experiment and observation in determining what exactly understanding consists of, what meaning consists of, and so on. The mental life we talk about so unquestioningly is very different from the mental life we actually have.