In the CTMU, tautology is used in a broad sense to refer to various kinds of logical necessity or circularity. In particular, Langan distinguishes three definitions of "tautology". In decreasing order of strength:
1. The self-referential sentential tautologies of 2VL;
2. Less general analytic statements like "daisies are flowers";
3. Any statement that is repetitive or redundant.
Statements meeting definition 1 above are called logical or syntactic tautologies, while those meeting definition 2 are called analytic or semantic tautologies. Syntactic tautologies are true by virtue of their logical form, while semantic tautologies are true by virtue of the meanings of their constituent terms.
- Langan, Christopher M. Chris Langan to Rick Rosner. Noesis No. 76. December 1992.