On Certainty (German: Über Gewissheit, original spelling Über Gewißheit) is a philosophical book composed from notes written by Ludwig Wittgenstein over four separate periods in the eighteen months before his death on 29 April 1951. He left his initial notes at the home of Elizabeth Anscombe, who linked them by theme with later passages in Wittgenstein's personal notebooks and (with G. H. von Wright), compiled them into a German/English parallel text book published in 1969. The translators were Denis Paul and Anscombe herself. (The editors also numbered and grouped the 676 passages; citations to the work are standardly given as OC1..OC676 rather than by page number.)

The book's concerns are largely epistemological, a recurrent theme being that there are some things which must be exempt from doubt in order for human practices to be possible, including the activity of raising doubts: "A doubt that doubted everything would not be a doubt" (OC450). The book takes as its starting point the 'here is one hand' argument made by G. E. Moore and examines the role of knowledge claims in human language, particularly of "certain ('gewisser') empirical propositions", what are now called Moorean propositions or Moorean certainties.

An important outcome is Wittgenstein's claim that all doubt is embedded in underlying beliefs and therefore the most radical forms of doubt must be rejected since they form a contradiction within the system that expressed them. Wittgenstein also sketched novel refutations of philosophical skepticism in various guises. Another recurrent motif (OC111,448,654), one that arguably unlocks the text for the lay reader, concerns the futility of endlessly re-checking an arithmetical calculation (OC77): what, precisely, is being re-checked? The calculation itself? Or, rather, the sanity, sobriety, and comprehension (say), of the re-checker? (See: Linguistic turn.) But (OC658): are not the sanity, sobriety, and comprehension of the re-checker presupposed by the very activity of, validly, checking and re-checking?