This lecture discusses the completeness of a proof system in classical propositional logic, focusing on deriving all propositional tautologies. It explores the soundness and refutational completeness theorems, resolution on clauses, conjunctive form, literals, and clauses, as well as unit resolution and transforming formulas into conjunctive normal form. The lecture also covers Tseytin's transformation, SAT solvers, the DPLL algorithm, data structures in a SAT solver, and generating simple proofs from SAT solver runs.