We prove completeness of the natural deduction proof calculus for propositional logic. 00:00 Introduction 02:12 Definition: reduced LP 03:13 Definition: Syntactic consistency 05:37 Lemma: If every syntactically consistent set has a model, then ND is complete. 12:48 Definition: Hintikka sets 16:38 Lemma: Every Hintikka set has a model 33:35 Lemma: Every syntactically consistent set can be extended to a Hintikka set 34:16 Claim: Enumerating rLP formulas 39:46 Constructing the extension 46:56 Claim: Delta is syntactically consistent 1:06:15 Claim: Delta is a Hintikka set 1:15:37 Completeness Theorem The playlist for the course can be found here: https://www.youtube.com/watch?v=0HImO-me_sg