Full Idea
Every sentence which occurs in the object language must also occur in the metalanguage, or can be translated into the metalanguage. There must also be logical terms, ...and semantic terms can only be introduced in the metalanguage by definition.
Gist of Idea
The metalanguage must contain the object language, logic, and defined semantics
Source
Alfred Tarski (The Semantic Conception of Truth [1944], 09)
Book Reference
'Semantics and the Philosophy of Language', ed/tr. Linsky,Leonard [University of Illinois 1972], p.22
A Reaction
He suggest that if the languages are 'typed', the meta-languag, to be 'richer', must contain variables of a higher logica type. Does this mean second-order logic?