Full Idea
I sort theories of truth into the large families of 'typed' and 'type-free'. Roughly, typed theories prohibit a truth predicate's application to sentences with occurrences of that predicate, and one cannot prove the truth of sentences containing 'true'.
Gist of Idea
Theories of truth are 'typed' (truth can't apply to sentences containing 'true'), or 'type-free'
Source
Volker Halbach (Axiomatic Theories of Truth [2011], II Intro)
Book Reference
Halbach,Volker: 'Axiomatic Theories of Truth' [CUP 2011], p.51
A Reaction
The problem sentence the typed theories are terrified of is the Liar Sentence. Typing produces a hierarchy of languages, referring down to the languages below them.
Related Idea
Idea 6006 If you say truly that you are lying, you are lying [Eubulides, by Dancy,R]