Full Idea
Tarski's distinction between object and metalanguage forms the basis of model theory.
Gist of Idea
The object language/ metalanguage distinction is the basis of model theory
Source
report of Alfred Tarski (The Concept of Truth for Formalized Languages [1933]) by Volker Halbach - Axiomatic Theories of Truth 11
Book Reference
Halbach,Volker: 'Axiomatic Theories of Truth' [CUP 2011], p.146