Full Idea
In a 'semantically closed' language all sentences which determine the adequate usage of 'true' can be asserted in the language. ...We can't change our logic, so we reject such languages. ...So must use two different languages to discuss truth.
Gist of Idea
We can't use a semantically closed language, or ditch our logic, so a meta-language is needed
Source
Alfred Tarski (The Semantic Conception of Truth [1944], 08-09)
Book Reference
'Semantics and the Philosophy of Language', ed/tr. Linsky,Leonard [University of Illinois 1972], p.20
A Reaction
This section explains why a meta-language is required. It rests entirely on the existence of the Liar paradox is a semantically closed language.