**Tautological forms - a molecular wff that that consists of tautological atomic propositions
logical necessity - true in virtue of its logical vocabulary - all cats are cats ← good
tautology - true in virtue of its propositional connectives - all cats are cats ← not good
15 - 16 chapters - tautological entailment - the most important chapter of the course
tautological validity - if all the premises the true, the conclusion must be true as well