Full Idea
People have not been aware that the language about which we speak need by no means coincide with the language in which we speak. ..But the language which contains its own semantics must inevitably be inconsistent.
Gist of Idea
A language containing its own semantics is inconsistent - but we can use a second language
Source
Alfred Tarski (The Establishment of Scientific Semantics [1936], p.402)
Book Reference
Tarski,Alfred: 'Logic, Semantics, Meta-mathematics' [Hackett 1956], p.402
A Reaction
It seems that Tarski was driven to propose the metalanguage approach mainly by the Liar Paradox.