Logic For Dummies, 2nd Edition

Chapter 11

Making Big Assumptions with Conditional and Indirect Proof

IN THIS CHAPTER

Grasping the basics of conditional proof

Assuming and discharging premises

Validating arguments with indirect proof

Mixing conditional and indirect proof

Have you ever seen one of those cheesy infomercials where the announcer keeps asking, “Now how much would you pay?” as they throw in a bunch of extra stuff you don’t want with your $19.95 oven-safe goggles — the cheese grater, the ice crusher, and the potato peeler that glows in the dark. This chapter is a bit like those commercials, but with one key difference: I throw in all sorts of extra stuff that you really do want.

In this chapter, I introduce two new methods of proof — conditional proof and indirect proof — completely free of charge. Unlike direct proof, which I discuss in Chapters 9 and 10, conditional proof and indirect proof involve an assumption, which is an additional premise you don’t know is true but you assume to be true. Also in this chapter, at no further cost to you, I show you not only how but also when to use conditional and ...