Full Idea
Neither logical nor mathematical propositions say anything about the world, because in virtue of their always being true they are consistent with any way the world could happen to be.
Gist of Idea
Logic and maths can't say anything about the world, since, as tautologies, they are consistent with all realities
Source
report of Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921]) by A.C. Grayling - Wittgenstein Ch.2
Book Reference
Grayling,A.C.: 'Wittgenstein' [OUP 2001], p.38
A Reaction
This became the standard view for twentieth century empiricists, and appeared to rule out a priori synthetic knowledge forever. Kripke's proposal that there are a posteriori necessities, however, changes the picture.