Therefore, no sentence of the object language can say of itself that it is (not) Tr, blocking the self-referential structure required by the Liar paradox.

The reason why this cannot be reproduced for Tr is that this predicate is always a predicate of a language (the metalanguage) different from the language of the sentences to which it applies (the object language). It is not possible to form a sentence of the language for which one defines Tr that says of itself that it is (not) Tr, since Tr is not a predicate of that language. On the other hand, it is certainly possible in some cases to form a sentence S of the metalanguage that says of itself t