In Chapter 8 we were concerned with the contrasts between deductive and inductive validity, and to that end we needed to give some account of deductive proof and disproof. In this chapter, the focus is exclusively on deduction; and we take the liberty of repeating some of the earlier parts of our exposition of the basics of logic, since they are so important, and can never suffer from repetition. It also means that this chapter can be read independently of Chapter 8.