ABSTRACT
On Certainty is comprised of notes written by Wittgenstein in the last two years of his life, in reaction to some papers by G. E. Moore on the subject of certainty and scepticism about the existence of external objects. In his 'Proof of an External World', Moore took it upon himself to prove the existence of external objects. This proof began with an act, the act of showing his hand, and this purported to be a display of knowledge. On Wittgenstein's view, languages are rule-governed, but the rules that govern them are not metalinguistic norms that exist in advance of use. Learning the meaning of a word is nothing but learning how it is used – that is, the conditions for its use: its grammar. In Philosophical Grammar, Wittgenstein thus defines 'what belongs to grammar': What belong to grammar are all the conditions necessary for comparing the proposition with reality.
