The lecture defines and differentiates interest, discount, and premium, treating their distinct meanings and how each is recorded in bookkeeping. It separates three common uses of discount — bank discount on early payment, trade cash deductions, and exchange adjustments for foreign remittances — and explains how true discount is computed by proportion or rule of three. It examines premium as an added charge or bounty, and analyzes usury as the rent of capital, showing why legal caps on rates arose and how customary practices can create effective higher rates. Historical attitudes toward lending and capital accumulation and practical ledger examples illustrate the economic and legal implications.