In some respects Wittgenstein made significant breaks with the empiricist tradition, especially in his views about language and the explanation of the rigour of the deductive sciences. His treatment of the relationship between mental events and physical events also represents an important departure. Empiricists generally have started from the important assumption that what a person is immediately acquainted with is his own sensations, ideas, and volitions; that these are mental and not physical; and, most important, that the things he knows immediately are essentially private and inaccessible to others. For both Moore and Russell there then arose the problem of how, in view of the privacy stressed by the sense-datum theory, the world of physical objects could be known. Wittgenstein’s attack on this viewpoint, which has come to be known as the “private language” argument, has been much discussed, partly because it was in this area that Wittgenstein presented what could most easily be identified as a more or less formal argument—one that could then be analyzed and criticized in an analytic manner. Even in this case, however, his style of writing was such that the proper formulation of the argument has become a main source of controversy. Wittgenstein argued that the notion of an utterly private experience would imply: (1) that what goes on in the mental life of a person could be talked about only in a language that that person alone could understand; (2) that such a private language would be no language at all (this has been the main source of controversy); and (3) that the widely held doctrine that there are absolutely private mental events cannot be intelligibly stated, because to say that there are such events is to speak in a public language about things that supposedly can be referred to only in a private language understandable by just one person.

The fact that Wittgenstein’s argument against private language depends essentially on the question “What is it to follow a rule?” illustrates a common characteristic of his writings, viz., that themes developed in one area of philosophy continually emerge in apparently quite divorced areas. His extraordinary ability to see a common source of difficulty in philosophical problems that seem to be unrelated helps to explain his style of writing, which seems at first sight to be a somewhat chaotic arrangement of ideas.

For a time, analytic philosophy was attracted to a behaviouristic view of mental phenomena according to which apparently private mental events, such as the feeling of fear, are not really private and in fact are definable in terms of publicly observable patterns of behaviour. Empiricism’s orientation toward science, which is founded on observation, together with the view that the evidence one has of what goes on in the mental lives of other people must derive from what one sees of their behaviour, has often warred against another inclination of empiricism, which is to regard the starting point of all knowledge of the world, for each person, as being essentially private sense experience. Wittgenstein was tremendously influential, however, in suggesting that these two extremes are not the only alternatives. Yet attempts to state how Wittgenstein could deny the privacy of experience without espousing some form of behaviourism—which treats emotions, desires, and attitudes as dispositions to behave in certain ways—have not been very successful. Sympathetic interpreters have taken up the notion of “criteria,” which Wittgenstein used but did not develop in any detail. The idea is that, for mental states such as fear, outward behaviour (e.g., running away, blanching, or cringing) does not constitute what it is to be in that state, as behaviourism would have it, but neither is such behaviour merely evidence of some completely private event. The problem has been to characterize the relation between behaviour and mental states in such a way that the two are neither identical nor evidence for each other while still allowing that knowledge of a person’s characteristic behaviour is essential to understanding the notion of a certain mental state.

The “therapeutic” function of philosophy

For the later Wittgenstein and many philosophers influenced by him, the proper role of philosophy is not, as it was for Russell, to develop theories in answer to philosophical problems but to clear up the conceptual confusions through which philosophical problems arise in the first place. These confusions invariably come about through misunderstandings of the complicated ways in which terms with philosophical import—such as know, believe, desire, intend, and think—are used in everyday life. Philosophers who are thus “bewitched” by language have been led to wonder, for example, how one can know what is going on in another’s mind or how desires and emotions can produce physical changes in the body, and vice versa. Examination of the actual workings of psychological language would, on this way of looking at philosophy, “dissolve” rather than solve the problems, for it would reveal features of the psychological concepts involved that philosophers, in their original formulation of the problems, had ignored or misunderstood. Philosophy is thus not an avenue to discovering philosophical truths but a kind of conceptual “therapy.” As Wittgenstein observed in the Philosophical Investigations (1953), the aim of philosophy is “to shew the fly the way out of the fly-bottle.”

Critics have argued that this way of looking at philosophy reduces the discipline to a sterile, inward-looking, and ultimately uninteresting enterprise. However, the confusions that philosophy, thus conceived, seeks to clear up need not be only those of philosophers. Scientists, for example, sometimes produce or presuppose philosophical theories that affect how they conduct their research—which, therefore, may be a fitting subject for philosophical therapeutics. Behaviourism in psychology seems to presuppose a philosophical theory and perhaps to be based on a general confusion about psychological concepts. More recently, some philosophers have suggested that contemporary cognitive science—and in particular the field of artificial intelligence, which views the human mind as a kind of computer—also is based on conceptual confusions created in large part by misunderstandings of the complexities of psychological speech. On this view, therefore, philosophy can have a therapeutic value beyond the sphere of philosophy itself.