This is Wittgenstein's posthumous book. The original German is given side by side with the English translation by G.E.M. Anscombe, which has undergone many corrections for this edition. Philosophical Investigations, like the Tractatus Logico-Philosophicus of 1918, is unconventionally organized. There are no chapters and no subheadings. Each numbered paragraph tells its own story. Large blocks of paragraphs deal with a single topic. For instance, the first thirty-eight paragraphs of Part I deal with the question of meaning. A given theme is treated at some length, dropped and is picked up again later on and in connection with another problem. This, plus Wittegnstein's unorthodox views may make the book difficult reading.
Wittgenstein's chief philosophical principle is that there are no philosophical problems. There are only philosophical muddles engendered by inattention to the proper uses of linguistic expressions. All of his main discussions in the book are general questions about language; not that language is the subject matter of philosophy but rather that an important, but not the only, function of philosophy is to clear away philosophical puzzles by tracing them to their source in linguistic muddles. Beyond the therapy lies the possibility of proposing different ways of talking, each of which, insofar as it is free of linguistic puzzles, may be a profitable way of looking at things analogous to "a new way of painting..." (p. 128, paragraph 401).
Wittgenstein's therapeutic method is best understood by seeing it in use. However, an inadequate idea of it may be conveyed by means of a general characterization. In the space allotted, I can do no more. To understand a linguistic expression in a given context describe the way(s) in which that expression functions in that context. Context is, in the last analysis, social context because languages that communicate, i.e., languages that are languages, cannot be private. This is not an empirical hypothesis but a statement of logical necessity. In the philosophy of psychology, this thesis is usually called logical behaviorism. Another way of putting Wittgenstein's general therapeutic prescription is this. To learn the "proper" meaning of a linguistic expression, investigate the ways in which we would learn or teach the use of the expression in specific contexts. We must pay particular attention to the ways in which the learner could get the wrong ideas about how to use the expression. We must also remember that the same utterance may function in many different kinds of contexts. Wittgenstein takes great pains to show the rich variety of usages. Clarifying meanings can be done only within an already existing language. This principle relates not to the ways in which language comes into existence but rather to the ways in which it functions as a means of communication.
The question of meaning in a way underlies every other question in the book. Here is a partial list of the many philosophical problems discussed: meaning, use and understanding; logical behaviorism and its consequences for the conception of philosophical analysis; thoughts, things and words; states of mind and conduct (as against involuntary action); sameness and difference of meaning, induction, deduction, memory. One would have to write such an extensive article even to begin exploring the method and cogency of Wittgenstein's philosophizing on these questions.
When one mentions philosophical analysis nowadays, Russell, Moore and Wittgenstein come to mind as the three fountainheads of three important 20th-Century styles of philosophical analysis. Wittgenstein's influence on Oxford philosophers and through them, and also directly, his influence on some 20th-Century American philosophers is enormous. These philosophers have used, though by no means slavishly, the Wittgenstein way of doing philosophy and their work is very suggestive. One need not agree with one's philosophical colleagues in order to admire the quality of their work. Suggestive philosophical processes and products, even if alien to our own ways of doing philosophy are, unless prima facie absurd, oftentimes more stimulating than agreement. For this reason, if for no other, Wittgenstein and the Wittgensteineans deserve serious attention.