Full Idea
Any affirmation can be negated: if it has sense to say p it also has sense to say ¬p. ...A proposition therefore is any expression which can be significantly negated.
Gist of Idea
A proposition is any expression which can be significantly negated
Source
Ludwig Wittgenstein (Lectures 1930-32 (student notes) [1931], B I.2)
Book Reference
Wittgenstein,Ludwig: 'Lectures in Cambridge 1930-32', ed/tr. Lee,Desmond [Blackwell 1980], p.22
A Reaction
I'm not sure about 'therefore'. I'm thinking you would have to already grasp the proposition in order to apply his negation test.