Wittgenstein’s last writings were on the subject of certainty. He wrote in response to G.E. Moore’s attack on skepticism about the external world. Moore had held up one hand, said “Here is one hand,” then held up his other hand and said “and here is another.” His point was that things outside the mind really do exist, we know they do, and that no grounds for skepticism could be strong enough to undermine this commonsense knowledge.
Wittgenstein did not defend skepticism, but questioned Moore’s claim to know that he had two hands. Such ‘knowledge’ is not something that one is ever taught, or finds out, or proves. It is more like a background against which we come to know other things. Wittgenstein compares this background to the bed of a river. This river bed provides the support, the context, in which claims to know various things have meaning. The bed itself is not something we can know or doubt. In normal circumstances no sane person doubts how many hands he or she has. But unusual circumstances can occur and what was part of the river bed can shift and become part of the river. I might, for instance, wake up dazed after a terrible accident and wonder whether my hands, which I cannot feel, are still there or not. This is quite different, though, from Descartes’s pretended doubt as to whether he has a body at all. Such radical doubt is really not doubt at all, from Wittgenstein’s point of view. And so it cannot be dispelled by a proof that the body exists, as Moore tried to do.
Recently some scholars have found in these ideas an additional stage in Wittgenstein’s development as a philosopher beyond that of the Philosophical Investigations. This “third Wittgenstein” no longer eschews theories and arguments, rather he develops a new argument against skepticism. This argument is foundational. According to it, language requires a certain form of life, which in turn requires living beings and the physical environment necessary to sustain them. Any doubt can only be expressed in language, and therefore presupposes or implies the existence of these requirements. They are the foundation of all our knowledge. To doubt their existence is to imply in language that the very requirements for language might be absent. This can only be a mistake.
Critics of this view suggest that if parts of the allegedly foundational river bed can become part of the river itself then the foundation metaphor is not really apt. The planet Earth is not logically necessary for human language, and the human form of life itself might change. So if our form of life and the environment needed to keep it going are foundational, then the foundation in question is far from stable. Nor does anyone claim that Wittgenstein’s foundationalism, if there is such a thing, is like that of other foundationalists in treating the indubitability of certain experiences or the truth of certain propositions as foundational. Instead, what Wittgenstein is supposed to regard as foundational is a certain form (or forms) of life, or the physical environment needed for such a form of life. Finally, while expressions of doubt might indeed require language, language itself consists in such things as expressions of doubt. Without any doubting our very form of life might no longer be recognizable as such. If that is the case then it seems potentially misleading to call our language or our form of life a necessary precondition, or foundation, for expressions of doubt. They seem to be parts of it more than distinct entities built upon it.