Chapter 4 Chapter Summaries

CHAPTER 4 SUMMARY

Common Forms

• Since argument forms are structures distinct from argument content, we can easily signify different forms by using letters to represent statements in the arguments. For example: If p, then q; p; Therefore, q. Some of the more common argument patterns that you encounter are like this pattern—they’re deductive, and they contain one or more conditional, or if-then, premises. The first statement in a conditional premise (the if part) is known as the antecedent. The second statement (the then part) is known as the consequent.

• There are several common patterns of valid conditional arguments: affirming the antecedent (modus ponens); denying the consequent (modus tollens); hypothetical syllogism (three conditional statements); disjunctive syllogism.

• There are two common patterns of invalid conditional arguments: affirming the consequent; denying the antecedent.

Reductio Ad Absurdum

• One kind of powerful argument that you frequently encounter is known as reductio ad absurdum (reduction to absurdity). The idea behind it is that if the contradictory (negation) of a statement leads to an absurdity or falsehood, then the negation of the statement is false and the statement itself must be true.