In his philosophical writings Wittgenstein was mainly concerned with questions concerning language and its various uses. But he was also always aware of the fact that any account concerning the limits of meaningful applications of language has an impact on the limits and/or the foundations of what can be known (compare, for instance, TLP, 5.5561, 5.6, 6.51, 6.53 with OC, 80, 114, 369-70, 514, 528). As he never questioned the possibility of knowledge, his critical attitude toward traditional philosophical theories and problems included a skeptical attitude toward skepticism as well. This became obvious in particular in his notes of 1949-51 which have been compiled and published under the title On Certainty.

Due to an unhappily written preface by the editors of that text, many readers have come to believe that Wittgenstein admired G. E. Moore's Defense of Common Sense and Proof of an External World and that he was commenting in his notes on these two papers with the intention of showing Moore to have been right in his philosophical attitude, but wrong in the way he argued for it. This is, however, not the case. Norman Malcolm reports1 that while Wittgenstein liked Moore as a decent man and felt stimulated by “Moore's Paradox” (cf. PI, pp. 190-92), he was not at all impressed by Moore's attempts to refute or reject idealism and/or skepticism.